In
medicine
Medicine is the science and practice of caring for a patient, managing the diagnosis, prognosis, prevention, treatment, palliation of their injury or disease, and promoting their health. Medicine encompasses a variety of health care pract ...
,
public health
Public health is "the science and art of preventing disease, prolonging life and promoting health through the organized efforts and informed choices of society, organizations, public and private, communities and individuals". Analyzing the det ...
, and
biology
Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary i ...
, transmission is the passing of a
pathogen
In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...
causing
communicable disease
An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable dise ...
from an infected
host
A host is a person responsible for guests at an event or for providing hospitality during it.
Host may also refer to:
Places
* Host, Pennsylvania, a village in Berks County
People
*Jim Host (born 1937), American businessman
* Michel Host ...
individual or group to a particular individual or group, regardless of whether the other individual was previously infected. The term strictly refers to the transmission of
microorganism
A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
s directly from one individual to another by one or more of the following means:
*
airborne transmission
Airborne or aerosol transmission is transmission of an infectious disease through small particles suspended in the air. Infectious diseases capable of airborne transmission include many of considerable importance both in human and veterinary ...
– very small dry and wet particles that stay in the air for long periods of time allowing airborne contamination even after the departure of the host. Particle size < 5
μm.
*
droplet transmission
A respiratory droplet is a small aqueous droplet produced by exhalation, consisting of saliva or mucus and other matter derived from respiratory tract surfaces. Respiratory droplets are produced naturally as a result of breathing, speaking, sn ...
– small and usually wet particles that stay in the air for a short period of time. Contamination usually occurs in the presence of the host. Particle size > 5 μm.
* direct physical contact – touching an infected individual, including sexual contact
* indirect physical contact – usually by touching a contaminated surface, including
soil
Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former te ...
(
fomite
A fomite () or fomes () is any inanimate object that, when contaminated with or exposed to infectious agents (such as pathogenic bacteria, viruses or fungi), can transfer disease to a new host.
Transfer of pathogens by fomites
A fomite is any ina ...
)
*
fecal–oral transmission – usually from unwashed hands, contaminated food or water sources due to lack of
sanitation
Sanitation refers to public health conditions related to clean drinking water and treatment and disposal of human excreta and sewage. Preventing human contact with feces is part of sanitation, as is hand washing with soap. Sanitation systems ...
and
hygiene
Hygiene is a series of practices performed to preserve health.
According to the World Health Organization (WHO), "Hygiene refers to conditions and practices that help to maintain health and prevent the spread of diseases." Personal hygiene refer ...
, an important transmission route in pediatrics, veterinary medicine and developing countries.
Transmission can also be indirect, via another
organism
In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and ...
, either a
vector
Vector most often refers to:
*Euclidean vector, a quantity with a magnitude and a direction
*Vector (epidemiology), an agent that carries and transmits an infectious pathogen into another living organism
Vector may also refer to:
Mathematic ...
(e.g. a
mosquito
Mosquitoes (or mosquitos) are members of a group of almost 3,600 species of small flies within the family Culicidae (from the Latin ''culex'' meaning " gnat"). The word "mosquito" (formed by ''mosca'' and diminutive ''-ito'') is Spanish for "li ...
or fly) or an intermediate host (e.g.
tapeworm in pigs can be transmitted to humans who ingest improperly cooked
pork
Pork is the culinary name for the meat of the domestic pig (''Sus domesticus''). It is the most commonly consumed meat worldwide, with evidence of pig husbandry dating back to 5000 BCE.
Pork is eaten both freshly cooked and preserved; ...
). Indirect transmission could involve
zoonoses
A zoonosis (; plural zoonoses) or zoonotic disease is an infectious disease of humans caused by a pathogen (an infectious agent, such as a bacterium, virus, parasite or prion) that has jumped from a non-human (usually a vertebrate) to a human. ...
or, more typically, larger pathogens like
macroparasite
Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has c ...
s with more complex
life cycle
Life cycle, life-cycle, or lifecycle may refer to:
Science and academia
*Biological life cycle, the sequence of life stages that an organism undergoes from birth to reproduction ending with the production of the offspring
*Life-cycle hypothesis, ...
s. Transmissions can be
autochthonous
Autochthon, autochthons or autochthonous may refer to:
Fiction
* Autochthon (Atlantis), a character in Plato's myth of Atlantis
* Autochthons, characters in the novel ''The Divine Invasion'' by Philip K. Dick
* Autochthon, a Primordial in the ' ...
(i.e. between two individuals in the same place) or may involve travel of the microorganism or the affected hosts.
Definition and related terms
An
infectious disease
An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable dise ...
agent can be transmitted in two ways: as disease agent transmission from one individual to another in the same generation (peers in the same age group) by either direct contact (licking, touching, biting), or indirect contact through air – cough or sneeze (
vectors or
fomites
A fomite () or fomes () is any inanimate object that, when contaminated with or exposed to infectious agents (such as pathogenic bacteria, viruses or fungi), can transfer disease to a new host.
Transfer of pathogens by fomites
A fomite is any inan ...
that allow the transmission of the agent causing the disease without physical contact)
or by
vertical disease transmission
A vertically transmitted infection is an infection caused by pathogenic bacteria or viruses that use mother-to-child transmission, that is, transmission directly from the mother to an embryo, fetus, or baby during pregnancy or childbirth. It ca ...
, passing the agent causing the disease from parent to offspring, such as in prenatal or
perinatal transmission
A vertically transmitted infection is an infection caused by pathogenic bacteria or viruses that use mother-to-child transmission, that is, transmission directly from the mother to an embryo, fetus, or baby during pregnancy or childbirth. It ca ...
.
The term
infectivity
In epidemiology, infectivity is the ability of a pathogen to establish an infection. More specifically, infectivity is a pathogen's capacity for horizontal transmission — that is, how frequently it spreads among hosts that are not in a parent ...
describes the ability of an
organism
In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and ...
to enter, survive and multiply in the host, while the ''infectiousness'' of a disease agent indicates the comparative ease with which the disease agent is transmitted to other hosts. Transmission of pathogens can occur by direct contact, through contaminated food, body fluids or objects, by airborne inhalation or through
vector
Vector most often refers to:
*Euclidean vector, a quantity with a magnitude and a direction
*Vector (epidemiology), an agent that carries and transmits an infectious pathogen into another living organism
Vector may also refer to:
Mathematic ...
organisms.
Transmissibility is the
probability
Probability is the branch of mathematics concerning numerical descriptions of how likely an Event (probability theory), event is to occur, or how likely it is that a proposition is true. The probability of an event is a number between 0 and ...
of an infection, given a contact between an infected host and a noninfected host.
Community transmission means that the source of infection for the spread of an illness is unknown or a link in terms of contacts between patients and other people is missing. It refers to the difficulty in grasping the epidemiological link in the community beyond confirmed cases.
Local transmission means that the source of the infection has been identified within the reporting location (such as within a country, region or city).
Routes of transmission
The route of transmission is important to
epidemiologists
Epidemiology is the study and analysis of the distribution (who, when, and where), patterns and determinants of health and disease conditions in a defined population.
It is a cornerstone of public health, and shapes policy decisions and evidenc ...
because patterns of contact vary between different populations and different groups of populations depending on socio-economic, cultural and other features. For example, low personal and food hygiene due to the lack of a clean water supply may result in increased transmission of diseases by the fecal-oral route, such as
cholera
Cholera is an infection of the small intestine by some strains of the bacterium ''Vibrio cholerae''. Symptoms may range from none, to mild, to severe. The classic symptom is large amounts of watery diarrhea that lasts a few days. Vomiting and ...
. Differences in incidence of such diseases between different groups can also throw light on the routes of transmission of the disease. For example, if it is noted that
polio
Poliomyelitis, commonly shortened to polio, is an infectious disease caused by the poliovirus. Approximately 70% of cases are asymptomatic; mild symptoms which can occur include sore throat and fever; in a proportion of cases more severe s ...
is more common in cities in underdeveloped countries, without a clean water supply, than in cities with a good plumbing system, we might advance the theory that polio is spread by the fecal-oral route. Two routes are considered to be
airborne
Airborne or Airborn may refer to:
Arts, entertainment, and media
Films
* ''Airborne'' (1962 film), a 1962 American film directed by James Landis
* ''Airborne'' (1993 film), a comedy–drama film
* ''Airborne'' (1998 film), an action film sta ...
: Airborne infections and droplet infections.
Airborne infection
"Airborne transmission refers to infectious agents that are spread via droplet nuclei (residue from evaporated droplets) containing infective microorganisms. These organisms can survive outside the body and remain suspended in the air for long periods of time. They infect others via the upper and lower respiratory tracts."
The size of the particles for airborne infections need to be < 5 μm.
It includes both dry and wet aerosols and thus requires usually higher levels of isolation since it can stay suspended in the air for longer periods of time. i.e., separate ventilation systems or negative pressure environments are needed to avoid general contamination. e.g.,
tuberculosis
Tuberculosis (TB) is an infectious disease usually caused by '' Mycobacterium tuberculosis'' (MTB) bacteria. Tuberculosis generally affects the lungs, but it can also affect other parts of the body. Most infections show no symptoms, in ...
,
chickenpox
Chickenpox, also known as varicella, is a highly contagious disease caused by the initial infection with varicella zoster virus (VZV). The disease results in a characteristic skin rash that forms small, itchy blisters, which eventually scab ...
,
measles
Measles is a highly contagious infectious disease caused by measles virus. Symptoms usually develop 10–12 days after exposure to an infected person and last 7–10 days. Initial symptoms typically include fever, often greater than , cough, ...
.
infection
![Sneeze](https://upload.wikimedia.org/wikipedia/commons/7/77/Sneeze.JPG)
A common form of transmission is by way of respiratory droplets, generated by
coughing
A cough is a sudden expulsion of air through the large breathing passages that can help clear them of fluids, irritants, foreign particles and microbes. As a protective reflex, coughing can be repetitive with the cough reflex following three phas ...
,
sneezing
A sneeze (also known as sternutation) is a semi-autonomous, convulsive expulsion of air from the lungs through the nose and mouth, usually caused by foreign particles irritating the nasal mucosa. A sneeze expels air forcibly from the mouth and ...
, or talking. Respiratory droplet transmission is the usual route for respiratory infections. Transmission can occur when respiratory droplets reach susceptible mucosal surfaces, such as in the eyes, nose or mouth. This can also happen indirectly via contact with
contaminated surfaces when hands then touch the face. Before drying, respiratory droplets are large and cannot remain suspended in the air for long, and are usually dispersed over short distances.
[ The size of the particles for droplet infections are > 5 μm.]
Organisms spread by droplet transmission include respiratory viruses such as influenza virus
''Orthomyxoviridae'' (from Greek ὀρθός, ''orthós'' 'straight' + μύξα, ''mýxa'' 'mucus') is a family of negative-sense RNA viruses. It includes seven genera: ''Alphainfluenzavirus'', ''Betainfluenzavirus'', '' Gammainfluenzavirus'', ...
, parainfluenza virus
Human parainfluenza viruses (HPIVs) are the viruses that cause human parainfluenza. HPIVs are a paraphyletic group of four distinct single-stranded RNA viruses belonging to the ''Paramyxoviridae'' family. These viruses are closely associated wit ...
, adenoviruses
Adenoviruses (members of the family ''Adenoviridae'') are medium-sized (90–100 nm), nonenveloped (without an outer lipid bilayer) viruses with an icosahedral nucleocapsid containing a double-stranded DNA genome. Their name derives from thei ...
, rhinovirus
The rhinovirus (from the grc, ῥίς, rhis "nose", , romanized: "of the nose", and the la, vīrus) is the most common viral infectious agent in humans and is the predominant cause of the common cold. Rhinovirus infection proliferates in tem ...
, respiratory syncytial virus
Respiratory syncytial virus (RSV), also called human respiratory syncytial virus (hRSV) and human orthopneumovirus, is a common, contagious virus that causes infections of the respiratory tract. It is a negative-sense, single-stranded RNA virus. ...
, human metapneumovirus
''Human metapneumovirus'' (HMPV) is a negative-sense single-stranded RNA virus of the family ''Pneumoviridae'' and is closely related to the ''Avian metapneumovirus'' (AMPV) subgroup C. It was isolated for the first time in 2001 in the Netherland ...
, ''Bordetella pertussis
''Bordetella pertussis'' is a Gram-negative, aerobic, pathogenic, encapsulated coccobacillus of the genus ''Bordetella'', and the causative agent of pertussis or whooping cough. Like '' B. bronchiseptica'', ''B. pertussis'' is motile and expres ...
'', pneumococci
''Streptococcus pneumoniae'', or pneumococcus, is a Gram-positive, spherical bacteria, alpha-hemolytic (under aerobic conditions) or beta-hemolytic (under anaerobic conditions), aerotolerant anaerobic member of the genus Streptococcus. They are ...
, streptococcus pyogenes
''Streptococcus pyogenes'' is a species of Gram-positive, aerotolerant bacteria in the genus ''Streptococcus''. These bacteria are extracellular, and made up of non-motile and non-sporing cocci (round cells) that tend to link in chains. They are ...
, diphtheria
Diphtheria is an infection caused by the bacterium '' Corynebacterium diphtheriae''. Most infections are asymptomatic or have a mild clinical course, but in some outbreaks more than 10% of those diagnosed with the disease may die. Signs and s ...
, rubella
Rubella, also known as German measles or three-day measles, is an infection caused by the rubella virus. This disease is often mild, with half of people not realizing that they are infected. A rash may start around two weeks after exposure and ...
, and coronaviruses
Coronaviruses are a group of related RNA viruses that cause diseases in mammals and birds. In humans and birds, they cause respiratory tract infections that can range from mild to lethal. Mild illnesses in humans include some cases of the com ...
. Spread of respiratory droplets from the wearer can be reduced through wearing of a surgical mask
A surgical mask, also known by other names such as a medical face mask or procedure mask, is a personal protective equipment used by healthcare professionals that serves as a mechanical barrier that interferes with direct airflow in and out of re ...
.
Direct contact
Direct contact occurs through skin-to-skin contact, kissing, and sexual intercourse. Direct contact also refers to contact with soil or vegetation harboring infectious organisms.[ ] Additionally, while fecal–oral transmission is primarily considered an indirect contact route, direct contact can also result in transmission through feces.
Diseases that can be transmitted by direct contact are called contagious (contagious is not the same as infectious; although all contagious diseases
An infection is the invasion of tissue (biology), tissues by pathogens, their multiplication, and the reaction of host (biology), host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmiss ...
are infectious, not all infectious diseases are contagious). These diseases can also be transmitted by sharing a towel (where the towel is rubbed vigorously on both bodies) or items of clothing in close contact with the body (socks, for example) if they are not washed thoroughly between uses. For this reason, contagious diseases often break out in schools, where towels are shared and personal items of clothing accidentally swapped in the changing rooms.
Some diseases that are transmissible by direct contact include athlete's foot
Athlete's foot, known medically as ''tinea pedis'', is a common skin infection of the feet caused by a fungus. Signs and symptoms often include itching, scaling, cracking and redness. In rare cases the skin may blister. Athlete's foot fungus m ...
, impetigo
Impetigo is a bacterial infection that involves the superficial skin. The most common presentation is yellowish crusts on the face, arms, or legs. Less commonly there may be large blisters which affect the groin or armpits. The lesions may be pa ...
, syphilis, wart
Warts are typically small, rough, hard growths that are similar in color to the rest of the skin. They typically do not result in other symptoms, except when on the bottom of the feet, where they may be painful. While they usually occur on the ...
s, and conjunctivitis
Conjunctivitis, also known as pink eye, is inflammation of the conjunctiva, outermost layer of the white part of the eye and the inner surface of the eyelid. It makes the eye appear pink or reddish. Pain, burning, scratchiness, or itchiness may ...
.
Sexual
This refers to any disease that can be caught during sexual activity with another person, including vaginal
In mammals, the vagina is the elastic, muscular part of the female genital tract. In humans, it extends from the vestibule to the cervix. The outer vaginal opening is normally partly covered by a thin layer of mucosal tissue called the hyme ...
or anal sex
Anal sex or anal intercourse is generally the insertion and thrusting of the erect penis into a person's anus, or anus and rectum, for sexual pleasure.Sepages 270–271for anal sex information, anpage 118for information about the clitoris. ...
or (less commonly) through oral sex
Oral sex, sometimes referred to as oral intercourse, is sexual activity involving the stimulation of the genitalia of a person by another person using the mouth (including the lips, tongue, or teeth) and the throat. Cunnilingus is oral sex per ...
(see below). Transmission is either directly between surfaces in contact during intercourse (the usual route for bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...
l infections and those infections causing sores) or from secretions (semen
Semen, also known as seminal fluid, is an organic bodily fluid created to contain spermatozoa. It is secreted by the gonads (sexual glands) and other sexual organs of male or hermaphroditic animals and can fertilize the female ovum. Semen i ...
or the fluid secreted by the excited female) which carry infectious agents that get into the partner's blood stream through tiny tears in the penis
A penis (plural ''penises'' or ''penes'' () is the primary sexual organ that male animals use to inseminate females (or hermaphrodites) during copulation. Such organs occur in many animals, both vertebrate and invertebrate, but males do n ...
, vagina
In mammals, the vagina is the elastic, muscular part of the female genital tract. In humans, it extends from the vestibule to the cervix. The outer vaginal opening is normally partly covered by a thin layer of mucosal tissue called the hymen ...
or rectum
The rectum is the final straight portion of the large intestine in humans and some other mammals, and the Gastrointestinal tract, gut in others. The adult human rectum is about long, and begins at the rectosigmoid junction (the end of the s ...
(this is a more usual route for virus
A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea.
Since Dmitri Ivanovsky's 1 ...
es). In this second case, anal sex is considerably more hazardous since the penis opens more tears in the rectum than the vagina, as the vagina is more elastic and more accommodating.
Some diseases transmissible by the sexual route include HIV/AIDS
Human immunodeficiency virus infection and acquired immunodeficiency syndrome (HIV/AIDS) is a spectrum of conditions caused by infection with the human immunodeficiency virus (HIV), a retrovirus. Following initial infection an individual ...
, chlamydia
Chlamydia, or more specifically a chlamydia infection, is a sexually transmitted infection caused by the bacterium ''Chlamydia trachomatis''. Most people who are infected have no symptoms. When symptoms do appear they may occur only several wee ...
, genital warts
Genital warts are a sexually transmitted infection caused by certain types of human papillomavirus (HPV). They are generally pink in color and project out from the surface of the skin. Usually they cause few symptoms, but can occasionally be pai ...
, gonorrhea
Gonorrhea, colloquially known as the clap, is a sexually transmitted infection (STI) caused by the bacterium '' Neisseria gonorrhoeae''. Infection may involve the genitals, mouth, or rectum. Infected men may experience pain or burning with ...
, hepatitis B
Hepatitis B is an infectious disease caused by the ''Hepatitis B virus'' (HBV) that affects the liver; it is a type of viral hepatitis. It can cause both acute and chronic infection.
Many people have no symptoms during an initial infection. Fo ...
, syphilis
Syphilis () is a sexually transmitted infection caused by the bacterium ''Treponema pallidum'' subspecies ''pallidum''. The signs and symptoms of syphilis vary depending in which of the four stages it presents (primary, secondary, latent, an ...
, herpes
Herpes simplex is a viral infection caused by the herpes simplex virus. Infections are categorized based on the part of the body infected.
Oral herpes involves the face or mouth. It may result in small blisters in groups often called cold s ...
, and trichomoniasis
Trichomoniasis (trich) is an infectious disease caused by the parasite ''Trichomonas vaginalis''. About 70% of affected people do not have symptoms when infected. When symptoms occur, they typically begin 5 to 28 days after exposure. Symptoms ca ...
.
Oral sexual
Sexually transmitted disease
Sexually transmitted infections (STIs), also referred to as sexually transmitted diseases (STDs) and the older term venereal diseases, are infections that are spread by sexual activity, especially vaginal intercourse, anal sex, and oral ...
s such as HIV and hepatitis B are thought to not normally be transmitted through mouth-to-mouth contact, although it is possible to transmit some STDs between the genitals and the mouth, during oral sex. In the case of HIV this possibility has been established. It is also responsible for the increased incidence of herpes simplex virus
Herpes simplex virus 1 and 2 (HSV-1 and HSV-2), also known by their taxonomical names ''Human alphaherpesvirus 1'' and '' Human alphaherpesvirus 2'', are two members of the human ''Herpesviridae'' family, a set of viruses that produce viral inf ...
1 (which is usually responsible for oral infections) in genital infections and the increased incidence of the type 2 virus (more common genitally) in oral infections.
Oral
Diseases that are transmitted primarily by oral means may be caught through direct oral contact such as kiss
A kiss is the touch or pressing of one's lips against another person or an object. Cultural connotations of kissing vary widely. Depending on the culture and context, a kiss can express sentiments of love, passion, romance, sexual attraction, ...
ing, or by indirect contact such as by sharing a drinking glass or a cigarette. Diseases that are known to be transmissible by kissing or by other direct or indirect oral contact include all of the diseases transmissible by droplet contact and (at least) all forms of herpes viruses, namely Cytomegalovirus
''Cytomegalovirus'' (''CMV'') (from ''cyto-'' 'cell' via Greek - 'container' + 'big, megalo-' + -''virus'' via Latin 'poison') is a genus of viruses in the order ''Herpesvirales'', in the family ''Herpesviridae'', in the subfamily ''Betaherpe ...
infections herpes simplex virus (especially HSV-1) and infectious mononucleosis
Infectious mononucleosis (IM, mono), also known as glandular fever, is an infection usually caused by the Epstein–Barr virus (EBV). Most people are infected by the virus as children, when the disease produces few or no symptoms. In young adult ...
.
Mother-to-child transmission
This is from mother to child (more rarely father to child), often ''in utero'', during childbirth
Childbirth, also known as labour and delivery, is the ending of pregnancy where one or more babies exits the internal environment of the mother via vaginal delivery or caesarean section. In 2019, there were about 140.11 million births globall ...
(also referred to as perinatal infection
A vertically transmitted infection is an infection caused by pathogenic bacteria or viruses that use mother-to-child transmission, that is, transmission directly from the mother to an embryo, fetus, or baby during pregnancy or childbirth. It can ...
) or during postnatal physical contact between parents and offspring. In mammals, including humans, it occurs also via breast milk
Breast milk (sometimes spelled as breastmilk) or mother's milk is milk produced by mammary glands located in the breast of a human female. Breast milk is the primary source of nutrition for newborns, containing fat, protein, carbohydrates ( lacto ...
(transmammary transmission). Infectious disease
An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable dise ...
s that can be transmitted in this way include: HIV, hepatitis B and syphilis. Many mutualistic organisms are transmitted vertically.
Iatrogenic
Transmission due to medical procedures, such as touching a wound, an injection
Injection or injected may refer to:
Science and technology
* Injective function, a mathematical function mapping distinct arguments to distinct values
* Injection (medicine), insertion of liquid into the body with a syringe
* Injection, in broadca ...
or transplantation of infected material. Some diseases that can be transmitted iatrogenic
Iatrogenesis is the causation of a disease, a harmful complication, or other ill effect by any medical activity, including diagnosis, intervention, error, or negligence. "Iatrogenic", ''Merriam-Webster.com'', Merriam-Webster, Inc., accessed 2 ...
ally include:
Creutzfeldt–Jakob disease
Creutzfeldt–Jakob disease (CJD), also known as subacute spongiform encephalopathy or neurocognitive disorder due to prion disease, is an invariably fatal degenerative brain disorder. Early symptoms include memory problems, behavioral changes, ...
by injection of contaminated human growth hormone
Growth hormone (GH) or somatotropin, also known as human growth hormone (hGH or HGH) in its human form, is a peptide hormone that stimulates growth, cell reproduction, and cell regeneration in humans and other animals. It is thus important in h ...
, MRSA
Methicillin-resistant ''Staphylococcus aureus'' (MRSA) is a group of Gram-positive bacteria that are genetically distinct from other strains of ''Staphylococcus aureus''. MRSA is responsible for several difficult-to-treat infections in humans. ...
and many more.
Indirect contact
Indirect contact transmission, also known as vehicleborne transmission, involves transmission through contamination
Contamination is the presence of a constituent, impurity, or some other undesirable element that spoils, corrupts, infects, makes unfit, or makes inferior a material, physical body, natural environment, workplace, etc.
Types of contamination
W ...
of inanimate objects. Vehicles that may indirectly transmit an infectious agent include food, water, biologic products such as blood
Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the c ...
, and fomite
A fomite () or fomes () is any inanimate object that, when contaminated with or exposed to infectious agents (such as pathogenic bacteria, viruses or fungi), can transfer disease to a new host.
Transfer of pathogens by fomites
A fomite is any ina ...
s such as handkerchiefs, bedding, or surgical scalpels. A vehicle may passively carry a pathogen, as in the case of food or water may carrying hepatitis A
Hepatitis A is an infectious disease of the liver caused by ''Hepatovirus A'' (HAV); it is a type of viral hepatitis. Many cases have few or no symptoms, especially in the young. The time between infection and symptoms, in those who develop them ...
virus. Alternatively, the vehicle may provide an environment in which the agent grows, multiplies, or produces toxin, such as improperly canned foods provide an environment that supports production of botulinum toxin
Botulinum toxin, or botulinum neurotoxin (BoNT), is a neurotoxic protein produced by the bacterium ''Clostridium botulinum'' and related species. It prevents the release of the neurotransmitter acetylcholine from axon endings at the neuromusc ...
by ''Clostridium botulinum
''Clostridium botulinum'' is a Gram-positive, rod-shaped, anaerobic, spore-forming, motile bacterium with the ability to produce the neurotoxin botulinum.
The botulinum toxin can cause botulism, a severe flaccid paralytic disease in humans an ...
''.
Transmission by other organisms
A ''vector
Vector most often refers to:
*Euclidean vector, a quantity with a magnitude and a direction
*Vector (epidemiology), an agent that carries and transmits an infectious pathogen into another living organism
Vector may also refer to:
Mathematic ...
'' is an organism
In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and ...
that does not cause disease itself but that transmits infection
An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable dise ...
by conveying pathogen
In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...
s from one host
A host is a person responsible for guests at an event or for providing hospitality during it.
Host may also refer to:
Places
* Host, Pennsylvania, a village in Berks County
People
*Jim Host (born 1937), American businessman
* Michel Host ...
to another.
Vectors may be mechanical or biological. A mechanical vector picks up an infectious agent on the outside of its body and transmits it in a passive manner. An example of a mechanical vector is a housefly
The housefly (''Musca domestica'') is a fly of the suborder Cyclorrhapha. It is believed to have evolved in the Cenozoic Era, possibly in the Middle East, and has spread all over the world as a commensal of humans. It is the most common fl ...
, which lands on cow dung, contaminating its appendages with bacteria from the feces, and then lands on food prior to consumption. The pathogen never enters the body of the fly. In contrast, biological vectors harbor pathogens within their bodies and deliver pathogens to new hosts in an active manner, usually a bite. Biological vectors are often responsible for serious blood-borne disease
A blood-borne disease is a disease that can be spread through contamination by blood and other body fluids. Blood can contain pathogens of various types, chief among which are microorganisms, like bacteria and parasites, and non-living infectious ...
s, such as malaria
Malaria is a mosquito-borne infectious disease that affects humans and other animals. Malaria causes symptoms that typically include fever, tiredness, vomiting, and headaches. In severe cases, it can cause jaundice, seizures, coma, or death. S ...
, viral encephalitis
Viral encephalitis is inflammation of the brain parenchyma, called encephalitis, by a virus. The different forms of viral encephalitis are called viral encephalitides. It is the most common type of encephalitis and often occurs with viral meningiti ...
, Chagas disease
Chagas disease, also known as American trypanosomiasis, is a tropical parasitic disease caused by ''Trypanosoma cruzi''. It is spread mostly by insects in the subfamily ''Triatominae'', known as "kissing bugs". The symptoms change over the cou ...
, Lyme disease
Lyme disease, also known as Lyme borreliosis, is a vector-borne disease caused by the ''Borrelia'' bacterium, which is spread by ticks in the genus ''Ixodes''. The most common sign of infection is an expanding red rash, known as erythema migran ...
and African sleeping sickness
African trypanosomiasis, also known as African sleeping sickness or simply sleeping sickness, is an insect-borne parasitic infection of humans and other animals. It is caused by the species '' Trypanosoma brucei''. Humans are infected by two ty ...
. Biological vectors are usually, though not exclusively, arthropod
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...
s, such as mosquito
Mosquitoes (or mosquitos) are members of a group of almost 3,600 species of small flies within the family Culicidae (from the Latin ''culex'' meaning " gnat"). The word "mosquito" (formed by ''mosca'' and diminutive ''-ito'') is Spanish for "li ...
es, tick
Ticks (order Ixodida) are parasitic arachnids that are part of the mite superorder Parasitiformes. Adult ticks are approximately 3 to 5 mm in length depending on age, sex, species, and "fullness". Ticks are external parasites, living by ...
s, flea
Flea, the common name for the order Siphonaptera, includes 2,500 species of small flightless insects that live as external parasites of mammals and birds. Fleas live by ingesting the blood of their hosts. Adult fleas grow to about long, a ...
s and lice
Louse ( : lice) is the common name for any member of the clade Phthiraptera, which contains nearly 5,000 species of wingless parasitic insects. Phthiraptera has variously been recognized as an order, infraorder, or a parvorder, as a result o ...
. Vectors are often required in the life cycle of a pathogen. A common strategy used to control vector-borne infectious diseases is to interrupt the life cycle of a pathogen by killing the vector.
Fecal–oral
In the fecal-oral route, pathogens
In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...
in fecal
Feces ( or faeces), known colloquially and in slang as poo and poop, are the solid or semi-solid remains of food that was not digested in the small intestine, and has been broken down by bacteria in the large intestine. Feces contain a relati ...
particles pass from one person to the mouth of another person. Although it is usually discussed as a route of transmission, it is actually a specification of the entry and exit portals of the pathogen, and can operate across several of the other routes of transmission. Fecal–oral transmission is primarily considered as an indirect contact route through contaminated food or water. However, it can also operate through direct contact with feces or contaminated body parts, such as through anal sex
Anal sex or anal intercourse is generally the insertion and thrusting of the erect penis into a person's anus, or anus and rectum, for sexual pleasure.Sepages 270–271for anal sex information, anpage 118for information about the clitoris. ...
. It can also operate through droplet or airborne transmission through the toilet plume
A toilet plume is the dispersal of microscopic particles as a result of flushing a toilet. Normal use of a toilet by healthy individuals is considered unlikely to be a major health risk. However this dynamic changes if an individual is fighting ...
from contaminated toilets.
Main causes of fecal–oral disease transmission include lack of adequate sanitation
Sanitation refers to public health conditions related to clean drinking water and treatment and disposal of human excreta and sewage. Preventing human contact with feces is part of sanitation, as is hand washing with soap. Sanitation systems ...
and poor hygiene
Hygiene is a series of practices performed to preserve health.
According to the World Health Organization (WHO), "Hygiene refers to conditions and practices that help to maintain health and prevent the spread of diseases." Personal hygiene refer ...
practices - which can take various forms. Fecal oral transmission can be via foodstuffs or water that has become contaminated. This can happen when people do not adequately wash their hands after using the toilet
A toilet is a piece of sanitary hardware that collects human urine and feces, and sometimes toilet paper, usually for disposal. Flush toilets use water, while dry or non-flush toilets do not. They can be designed for a sitting position popu ...
and before preparing food or tending to patients.
The fecal-oral route of transmission can be a public health risk for people in developing countries
A developing country is a sovereign state with a lesser developed industrial base and a lower Human Development Index (HDI) relative to other countries. However, this definition is not universally agreed upon. There is also no clear agreem ...
who live in urban slum
A slum is a highly populated urban residential area consisting of densely packed housing units of weak build quality and often associated with poverty. The infrastructure in slums is often deteriorated or incomplete, and they are primarily inh ...
s without access to adequate sanitation. Here, excreta
Excretion is a process in which metabolic waste
is eliminated from an organism. In vertebrates this is primarily carried out by the lungs, kidneys, and skin. This is in contrast with secretion, where the substance may have specific tasks after lea ...
or untreated sewage
Sewage (or domestic sewage, domestic wastewater, municipal wastewater) is a type of wastewater that is produced by a community of people. It is typically transported through a sewer system. Sewage consists of wastewater discharged from residenc ...
can pollute drinking water sources (groundwater
Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidate ...
or surface water). The people who drink the polluted water can become infected. Another problem in some developing countries, is open defecation
Open defecation is the human practice of defecating outdoors ("in the open") rather than into a toilet. People may choose fields, bushes, forests, ditches, streets, canals, or other open spaces for defecation. They do so either because they do ...
which leads to disease transmission via the fecal-oral route.
Even in developed countries there are periodic system failures resulting in a sanitary sewer overflow
Sanitary sewer overflow (SSO) is a condition in which untreated sewage is discharged from a sanitary sewer into the environment prior to reaching sewage treatment facilities. When caused by rainfall it is also known as wet weather overflow. Cause ...
. This is the typical mode of transmission for infectious agents such as cholera
Cholera is an infection of the small intestine by some strains of the bacterium ''Vibrio cholerae''. Symptoms may range from none, to mild, to severe. The classic symptom is large amounts of watery diarrhea that lasts a few days. Vomiting and ...
, hepatitis A
Hepatitis A is an infectious disease of the liver caused by ''Hepatovirus A'' (HAV); it is a type of viral hepatitis. Many cases have few or no symptoms, especially in the young. The time between infection and symptoms, in those who develop them ...
, polio
Poliomyelitis, commonly shortened to polio, is an infectious disease caused by the poliovirus. Approximately 70% of cases are asymptomatic; mild symptoms which can occur include sore throat and fever; in a proportion of cases more severe s ...
, Rotavirus
''Rotavirus'' is a genus of double-stranded RNA viruses in the family ''Reoviridae''. Rotaviruses are the most common cause of diarrhoeal disease among infants and young children. Nearly every child in the world is infected with a rotavirus a ...
, ''Salmonella
''Salmonella'' is a genus of rod-shaped (bacillus) Gram-negative bacteria of the family Enterobacteriaceae. The two species of ''Salmonella'' are ''Salmonella enterica'' and ''Salmonella bongori''. ''S. enterica'' is the type species and is fur ...
'', and parasite
Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has ...
s (e.g. ''Ascaris lumbricoides
''Ascaris lumbricoides'' is a large parasitic worm that causes ascariasis in humans. A Nematoda, roundworm of genus ''Ascaris'', it is the most common parasitic worm in humans. An estimated one-sixth of the human population is at some point infe ...
'').
Tracking
Tracking the transmission of infectious diseases is called disease surveillance
Disease surveillance is an epidemiological practice by which the spread of disease is monitored in order to establish patterns of progression. The main role of disease surveillance is to predict, observe, and minimize the harm caused by outbreak, ...
. Surveillance of infectious diseases in the public realm traditionally has been the responsibility of public health
Public health is "the science and art of preventing disease, prolonging life and promoting health through the organized efforts and informed choices of society, organizations, public and private, communities and individuals". Analyzing the det ...
agencies, on an international, national, or local level. Public health staff relies on health care workers and microbiology laboratories to report cases of reportable disease
A notifiable disease is any disease that is required by law to be reported to government authorities. The collation of information allows the authorities to monitor the disease, and provides early warning of possible outbreaks. In the case of live ...
s to them. The analysis of aggregate data
Aggregate data is high-level data which is acquired by combining individual-level data. For instance, the output of an industry is an aggregate of the firms’ individual outputs within that industry. Aggregate data are applied in statistics, da ...
can show the spread of a disease and is at the core of the specialty of epidemiology
Epidemiology is the study and analysis of the distribution (who, when, and where), patterns and determinants of health and disease conditions in a defined population.
It is a cornerstone of public health, and shapes policy decisions and evidenc ...
.
To understand the spread of the vast majority of non-notifiable diseases, data either need to be collected in a particular study, or existing data collections can be mined, such as insurance company data or antimicrobial drug sales for example.
For diseases transmitted within an institution, such as a hospital, prison, nursing home, boarding school, orphanage, refugee camp, etc., infection control
Infection prevention and control is the discipline concerned with preventing healthcare-associated infections; a practical rather than academic sub-discipline of epidemiology. In Northern Europe, infection prevention and control is expanded from ...
specialists are employed, who will review medical records to analyze transmission as part of a hospital epidemiology program, for example.
Because these traditional methods are slow, time-consuming, and labor-intensive, proxies of transmission have been sought. One proxy in the case of influenza is tracking of influenza-like illness
Influenza-like illness (ILI), also known as flu-like syndrome or flu-like symptoms, is a medical diagnosis of possible influenza or other illness causing a set of common symptoms. These include fever, shivering, chills, malaise, dry cough, lo ...
at certain sentinel sites of health care practitioners within a state, for example. Tools have been developed to help track influenza epidemic
An epidemic (from Ancient Greek, Greek ἐπί ''epi'' "upon or above" and δῆμος ''demos'' "people") is the rapid spread of disease to a large number of patients among a given population within an area in a short period of time.
Epidemics ...
s by finding patterns in certain web search query
A web query or web search query is a query that a user enters into a web search engine to satisfy their information needs. Web search queries are distinctive in that they are often plain text and boolean search directives are rarely used. They var ...
activity. It was found that the frequency of influenza-related web searches as a whole rises as the number of people sick with influenza rises. Examining space-time relationships of web queries has been shown to approximate the spread of influenza and dengue
Dengue fever is a mosquito-borne tropical disease caused by the dengue virus. Symptoms typically begin three to fourteen days after infection. These may include a high fever, headache, vomiting, muscle and joint pains, and a characteristic ...
.
Computer simulation
Computer simulation is the process of mathematical modelling, performed on a computer, which is designed to predict the behaviour of, or the outcome of, a real-world or physical system. The reliability of some mathematical models can be dete ...
s of infectious disease spread have been used.
Human aggregation can drive transmission, seasonal variation
In time series data, seasonality is the presence of variations that occur at specific regular intervals less than a year, such as weekly, monthly, or quarterly. Seasonality may be caused by various factors, such as weather, vacation, and holidays a ...
and outbreaks
In epidemiology, an outbreak is a sudden increase in occurrences of a disease when cases are in excess of normal expectancy for the location or season. It may affect a small and localized group or impact upon thousands of people across an entire ...
of infectious diseases, such as the annual start of school, bootcamp, the annual Hajj
The Hajj (; ar, حَجّ '; sometimes also spelled Hadj, Hadji or Haj in English) is an annual Islamic pilgrimage to Mecca, Saudi Arabia, the holiest city for Muslims. Hajj is a mandatory religious duty for Muslims that must be carried ...
etc. Most recently, data from cell phones have been shown to be able to capture population movements well enough to predict the transmission of certain infectious diseases, like rubella.
Relationship with virulence and survival
Pathogens must have a way to be transmitted from one host to another to ensure their species' survival. Infectious agents are generally specialized for a particular method of transmission. Taking an example from the respiratory route, from an evolutionary perspective viruses or bacteria that cause their host to develop coughing and sneezing symptoms have a great survival advantage, as they are much more likely to be ejected from one host and carried to another. This is also the reason that many microorganisms cause diarrhea
Diarrhea, also spelled diarrhoea, is the condition of having at least three loose, liquid, or watery bowel movements each day. It often lasts for a few days and can result in dehydration due to fluid loss. Signs of dehydration often begin wi ...
.
The relationship between virulence
Virulence is a pathogen's or microorganism's ability to cause damage to a host.
In most, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. The pathogenicity of an organism—its ability to ca ...
and transmission is complex and has important consequences for the long term evolution of a pathogen. Since it takes many generations for a microbe and a new host species to co-evolve, an emerging pathogen may hit its earliest victims especially hard. It is usually in the first wave of a new disease that death rates are highest. If a disease is rapidly fatal, the host may die before the microbe can be passed along to another host. However, this cost may be overwhelmed by the short-term benefit of higher infectiousness if transmission is linked to virulence, as it is for instance in the case of cholera (the explosive diarrhea aids the bacterium in finding new hosts) or many respiratory infections (sneezing and coughing create infectious aerosol
An aerosol is a suspension (chemistry), suspension of fine solid particles or liquid Drop (liquid), droplets in air or another gas. Aerosols can be natural or Human impact on the environment, anthropogenic. Examples of natural aerosols are fog o ...
s).
Anything that reduces the rate of transmission of an infection carries positive externalities
In economics, an externality or external cost is an indirect cost or benefit to an uninvolved third party that arises as an effect of another party's (or parties') activity. Externalities can be considered as unpriced goods involved in either co ...
, which are benefits to society that are not reflected in a price to a consumer. This is recognized implicitly when vaccines are offered for free or at a cost to the patient less than the purchase price.
Beneficial microorganisms
The mode of transmission is also an important aspect of the biology of beneficial microbial symbionts, such as coral
Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and sec ...
-associated dinoflagellates or human microbiota
This article lists some of the species recognized as belonging to the human microbiome.
Whole-body distributed
*''Acinetobacter calcoaceticus''
*'' Burkholderia cepacia''
*''Pseudomonas pseudoalcaligenes''
*''Peptostreptococcus spp''
Natur ...
. Organisms can form symbioses with microbes transmitted from their parents, from the environment or unrelated individuals, or both.
Vertical transmission
Vertical transmission Vertical transmission of symbionts is the transfer of a microbial symbiont from the parent directly to the offspring. Many metazoan species carry symbiotic bacteria which play a mutualistic, commensal, or parasitic role. A symbiont is acquire ...
refers to acquisition of symbionts from parents (usually mothers). Vertical transmission can be intracellular (e.g. transovarial), or extracellular (for example through post-embryonic contact between parents and offspring). Both intracellular and extracellular vertical transmission can be considered a form of non-genetic inheritance
Inheritance is the practice of receiving private property, Title (property), titles, debts, entitlements, Privilege (law), privileges, rights, and Law of obligations, obligations upon the death of an individual. The rules of inheritance differ ...
or parental effect. It has been argued that most organisms experience some form of vertical transmission of symbionts. Canonical examples of vertically transmitted symbionts include the nutritional symbiont Buchnera in aphids (transovarially transmitted intracellular symbiont) and some components of the human microbiota
This article lists some of the species recognized as belonging to the human microbiome.
Whole-body distributed
*''Acinetobacter calcoaceticus''
*'' Burkholderia cepacia''
*''Pseudomonas pseudoalcaligenes''
*''Peptostreptococcus spp''
Natur ...
(transmitted during passage of infants through the birth canal and also through breastfeeding).
Horizontal transmission
Some beneficial symbionts are acquired horizontally, from the environment or unrelated individuals. This requires that host and symbiont have some method of recognizing each other or each other's products or services. Often, horizontally acquired symbionts are relevant to secondary
Secondary may refer to: Science and nature
* Secondary emission, of particles
** Secondary electrons, electrons generated as ionization products
* The secondary winding, or the electrical or electronic circuit connected to the secondary winding i ...
rather than primary metabolism, for example for use in defense against pathogens, but some primary nutritional symbionts are also horizontally (environmentally) acquired. Additional examples of horizontally transmitted beneficial symbionts include bioluminescent bacteria associated with bobtail squid and nitrogen-fixing bacteria in plants.
Mixed-mode transmission
Many microbial symbionts, including human microbiota
This article lists some of the species recognized as belonging to the human microbiome.
Whole-body distributed
*''Acinetobacter calcoaceticus''
*'' Burkholderia cepacia''
*''Pseudomonas pseudoalcaligenes''
*''Peptostreptococcus spp''
Natur ...
, can be transmitted both vertically and horizontally. Mixed-mode transmission can allow symbionts to have the “best of both worlds” – they can vertically infect host offspring when host density is low, and horizontally infect diverse additional hosts when a number of additional hosts are available. Mixed-mode transmission make the outcome (degree of harm or benefit) of the relationship more difficult to predict, because the evolutionary success of the symbiont is sometimes but not always tied to the success of the host.
See also
* Bioaerosol Bioaerosols (short for biological aerosols) are a subcategory of particles released from terrestrial and marine ecosystems into the atmosphere. They consist of both living and non-living components, such as fungi, pollen, bacteria and viruses. Comm ...
* Bugchasing
Bugchasing (alternatively bug chasing) is the rare practice of intentionally seeking human immunodeficiency virus (HIV) infection through sexual activity.
Bugchasers—those who eroticize HIV—constitute a subculture of barebackers, men who ...
* Cross-species transmission
Cross-species transmission (CST), also called interspecies transmission, host jump, or spillover, is the transmission of an infectious pathogen, such as a virus, between hosts belonging to different species. Once introduced into an individual of a ...
* Infectious disease: Transmission
* Transmission coefficient (epidemiology) The transmission coefficient is needed for models of Transmission where for instance a vector carries a pathogen or parasite between hosts. It is represented by β and it is the rate at which a pathogen moves from infected individuals to susceptible ...
* Transmission risks and rates
Transmission of an infection requires three conditions:
*an infectious individual
*a susceptible individual
*an effective contact between them
An effective contact is defined as any kind of contact between two individuals such that, if one individ ...
References
{{Authority control
Human diseases and disorders