Human-to-human transmission (HHT) is an

epidemiologic 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 ...

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 ...

, especially in case the disease is borne by individuals known as

superspreaders A superspreading event (SSEV) is an event in which an infectious disease is spread much more than usual, while an unusually contagious organism infected with a disease is known as a superspreader. In the context of a human-borne illness, a super ...

. In these cases, the

basic reproduction number In epidemiology, the basic reproduction number, or basic reproductive number (sometimes called basic reproduction ratio or basic reproductive rate), denoted R_0 (pronounced ''R nought'' or ''R zero''), of an infection is the expected number of ...

of the virus, which is the average number of additional people that a single case will infect without any preventative measures, can be as high as 203.9. Interhuman transmission is a synonym for HHT. The

World Health Organization The World Health Organization (WHO) is a specialized agency of the United Nations responsible for international public health. The WHO Constitution states its main objective as "the attainment by all peoples of the highest possible level of h ...

designation of a

pandemic A pandemic () is an epidemic of an infectious disease that has spread across a large region, for instance multiple continents or worldwide, affecting a substantial number of individuals. A widespread endemic (epidemiology), endemic disease wi ...

hinges on the demonstrable fact that there is sustained HHT in two regions of the world.

Synopsis

Relevant

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 ...

may be

viruses 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 ...

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 ...

, or

fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...

, and they may be spread through breathing, talking, coughing, sneezing, spraying of liquids, toilet flushing or any activities which generate

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 ...

particles or droplets or generate

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 ...

, such as raising of dust. A 2007 study showed that influenza virus was still active on

stainless steel Stainless steel is an alloy of iron that is resistant to rusting and corrosion. It contains at least 11% chromium and may contain elements such as carbon, other nonmetals and metals to obtain other desired properties. Stainless steel's corros ...

24 hours after contamination. Though on hands it survives only for five minutes, the constant contact with steel almost certainly transmits infection. Transfer efficiency depends not only on surface, but also on pathogen type. For example,

avian influenza Avian influenza, known informally as avian flu or bird flu, is a variety of influenza caused by viruses adapted to birds.

survives on both porous and non-porous materials for 144 hours. The pathogens may also be transmitted by poor use of cutlery or improper sanitation of dishes or bedlinen. Particularly problematic are toilet practices, which lead to the fecal-oral route. STDs are by definition spread through this vector.

List of HHT diseases

Examples of some HHT diseases are listed below.

Controlled

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, ...

vaccine A vaccine is a biological Dosage form, preparation that provides active acquired immunity to a particular infectious disease, infectious or cancer, malignant disease. The safety and effectiveness of vaccines has been widely studied and verifie ...

available *

mumps MUMPS ("Massachusetts General Hospital Utility Multi-Programming System"), or M, is an imperative, high-level programming language with an integrated transaction processing key–value database. It was originally developed at Massachusetts Gener ...

: vaccine available *

chicken pox 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 ...

: vaccine available *

small pox Smallpox was an infectious disease caused by variola virus (often called smallpox virus) which belongs to the genus Orthopoxvirus. The last naturally occurring case was diagnosed in October 1977, and the World Health Organization (WHO) cer ...

bubonic plague Bubonic plague is one of three types of plague caused by the plague bacterium (''Yersinia pestis''). One to seven days after exposure to the bacteria, flu-like symptoms develop. These symptoms include fever, headaches, and vomiting, as well a ...

: slim non-nil risk *

pneumonic plague Pneumonic plague is a severe lung infection caused by the bacterium ''Yersinia pestis''. Symptoms include fever, headache, shortness of breath, chest pain, and coughing. They typically start about three to seven days after exposure. It is one ...

: 1910-11 Manchurian plague *

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 ...

Norovirus Norovirus, sometimes referred to as the winter vomiting disease, is the most common cause of gastroenteritis. Infection is characterized by non-bloody diarrhea, vomiting, and stomach pain. Fever or headaches may also occur. Symptoms usually deve ...

monkeypox Monkeypox (also called mpox by the WHO) is an infectious viral disease that can occur in humans and some other animals. Symptoms include fever, swollen lymph nodes, and a rash that forms blisters and then crusts over. The time from exposure t ...

SARS-CoV-1 Severe acute respiratory syndrome coronavirus 1 (SARS-CoV-1; or Severe acute respiratory syndrome coronavirus, SARS-CoV) is a strain of coronavirus that causes severe acute respiratory syndrome (SARS), the respiratory illness responsible for ...

MERS Middle East respiratory syndrome (MERS) is a viral respiratory infection caused by ''Middle East respiratory syndrome–related coronavirus'' (MERS-CoV). Symptoms may range from none, to mild, to severe. Typical symptoms include fever, cough, ...

Avian flu Avian influenza, known informally as avian flu or bird flu, is a variety of influenza caused by viruses adapted to birds.

Sexually transmitted infection Sexually transmitted infections (STIs), also referred to as sexually transmitted diseases (STDs) and the older term venereal diseases, are infections that are Transmission (medicine), spread by Human sexual activity, sexual activity, especi ...

s (STIs) or sexually transmitted diseases (STDs): **

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 ...

, aka French pox

Uncontrolled

SARS-CoV-2 Severe acute respiratory syndrome coronavirus 2 (SARS‑CoV‑2) is a strain of coronavirus that causes COVID-19 (coronavirus disease 2019), the respiratory illness responsible for the ongoing COVID-19 pandemic. The virus previously had a ...

References

Sources

* * {{Concepts in infectious disease Epidemiology Parasitology Infectious diseases Sanitation Hygiene Global health Epidemics