A skin infection is an

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 ...

of the

skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, animal coverings, such as the arthropod exoskeleton, have diffe ...

in humans and other animals, that can also affect the associated

soft tissue Soft tissue is all the tissue in the body that is not hardened by the processes of ossification or calcification such as bones and teeth. Soft tissue connects, surrounds or supports internal organs and bones, and includes muscle, tendons, ligam ...

s such as

loose connective tissue Loose connective tissue, sometimes called areolar tissue, is a cellular connective tissue with thin and relatively sparse collagen fibers. Its ground substance occupies more volume than the fibers do. It has a viscous to gel-like consistenc ...

and

mucous membrane A mucous membrane or mucosa is a membrane that lines various cavities in the body of an organism and covers the surface of internal organs. It consists of one or more layers of epithelial cells overlying a layer of loose connective tissue. It is ...

s. They comprise a category of infections termed

skin and skin structure infection Skin and skin structure infections (SSSIs), also referred to as skin and soft tissue infections (SSTIs), or acute bacterial skin and skin structure infections (ABSSSIs), are infections of skin and associated soft tissues (such as loose connective ti ...

s (SSSIs), or skin and soft tissue infections (SSTIs), and acute bacterial SSSIs (ABSSSIs). They are distinguished from

dermatitis Dermatitis is inflammation of the skin, typically characterized by itchiness, redness and a rash. In cases of short duration, there may be small blisters, while in long-term cases the skin may become thickened. The area of skin involved can v ...

(

inflammation Inflammation (from la, wikt:en:inflammatio#Latin, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or Irritation, irritants, and is a protective response involving im ...

of the skin), although skin infections can result in skin inflammation.

Causes

Bacterial

Bacterial skin infections affected about 155 million people and cellulitis occurred in about 600 million people in 2013. Bacterial skin infections include: *

Cellulitis Cellulitis is usually a bacterial infection involving the inner layers of the skin. It specifically affects the dermis and subcutaneous fat. Signs and symptoms include an area of redness which increases in size over a few days. The borders of t ...

, a diffuse

connective tissue Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...

with severe inflammation of

dermal The dermis or corium is a layer of skin between the epidermis (with which it makes up the cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from stress and strain. It is divided in ...

and subcutaneous layers of the

. Further, cellulitis can be classified based into purulent and non-purulent cellulitis, based on the most likely causative agent and the symptoms presentation. Purulent cellulitis is often caused by ''

Staphylococcus aureus ''Staphylococcus aureus'' is a Gram-positive spherically shaped bacterium, a member of the Bacillota, and is a usual member of the microbiota of the body, frequently found in the upper respiratory tract and on the skin. It is often positive ...

'', including both methicillin-sensitive (MSSA) and methicillin-resistant ''S. aureus'' (MRSA). Non-purulent cellulitis is most often associated with group A beta-hemolytic streptococci, such as ''Streptococcus pyogenes.'' In rare cases, the infection can progress into

necrotizing fasciitis Necrotizing fasciitis (NF), also known as flesh-eating disease, is a bacterial infection that results in the death of parts of the body's soft tissue. It is a severe disease of sudden onset that spreads rapidly. Symptoms usually include red or p ...

, a serious and potentially fatal infection. *

Erysipelas Erysipelas () is a relatively common bacterial infection of the superficial layer of the skin ( upper dermis), extending to the superficial lymphatic vessels within the skin, characterized by a raised, well-defined, tender, bright red rash, t ...

, is a bacterial infection which primarily effects superficial dermis, and often involves superficial lymphatics. Unlike cellulitis, it does not effect deeper layers of the skin. It is primarily caused by the Group A beta-hemolytic streptococci, with ''Streptococcus pyogenes'' being the most common pathogen. *

Folliculitis Folliculitis is the infection and inflammation of one or more hair follicles. The condition may occur anywhere on hair-covered skin. The rash may appear as pimples that come to white tips on the face, chest, back, arms, legs, buttocks, or head. A ...

, is a skin condition in which

hair follicle The hair follicle is an organ found in mammalian skin. It resides in the dermal layer of the skin and is made up of 20 different cell types, each with distinct functions. The hair follicle regulates hair growth via a complex interaction between h ...

, located in the dermal layer of the skin, becomes infected and inflamed. It is predominantly caused by bacterial infections, especially ''

'', leading to superficial bacterial folliculitis. Other causative agents of folliculitis include fungi (most commonly ''

Malassezia ''Malassezia'' (formerly known as ''Pityrosporum'') is a genus of fungi. It is the sole genus in family Malasseziaceae, which is the only family in order Malasseziales, itself the single member of class Malasseziomycetes. ''Malassezia'' species ...

'' species), viruses (such as

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 ...

), and mites (''

Demodex ''Demodex'' is a genus of tiny mites that live in or near hair follicles of mammals. Around 65 species of ''Demodex'' are known. Two species live on humans: ''Demodex folliculorum'' and ''Demodex brevis'', both frequently referred to as eyelash ...

'' species). *

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 ...

, a highly contagious ABSSSI (acute bacterial skin and skin structure infection) common among pre-school children, primarily associated with the pathogens '' S. aureus'' and '' S. pyogenes.'' Impetigo has a characteristic appearance with yellow (honey-coloured), crusted lesions occurring around mouth, nose, and chin. It is estimated, that at any given time, it effects 140 million people globally. Impetigo can be further classified into bullous and nonbullous forms. Nonbullous impetigo is the most common form, representing approximately 70% of diagnosed cases. The remaining 30% of cases represent bullous form, which is primarily caused by '' S. aureus.'' In rare instances, bullous impetigo can spread and lead to

Staphylococcal Scalded Skin Syndrome Staphylococcal scalded skin syndrome (SSSS) is a dermatological condition caused by ''Staphylococcus aureus''. Signs and symptoms The disease presents with the widespread formation of fluid-filled blisters that are thin walled and easily ruptured ...

(SSSS), a potentially life-threatening infection.

Fungal

Fungal skin infections may present as either a superficial or deep

of the skin, hair, and/or nails.

Mycetoma Mycetoma is a chronic infection in the skin caused by either bacteria (actinomycetoma) or fungi (eumycetoma), typically resulting in a triad of painless firm skin lumps, the formation of weeping sinuses, and a discharge that contains grains. 80% ...

are a broad group of fungal infections that characteristically originate in the skin and subcutaneous tissues of the foot. If not treated appropriately and in a timely fashion mycetoma infections can extend to deeper tissues like bones and joints causing osteomyelitis. Extensive osteomyelitis can necessitate surgical bone resections and even lower limb amputation. As of 2010, they affect about one billion people globally. Some examples of common fungal skin infections include: *

Dermatophytosis Dermatophytosis, also known as ringworm, is a fungal infection of the skin. Typically it results in a red, itchy, scaly, circular rash. Hair loss may occur in the area affected. Symptoms begin four to fourteen days after exposure. Multiple ar ...

, also known as ringworm, is a superficial fungal infection of the skin caused by several different species of fungi. The fungal genera which cause skin infections in humans include ''

Trichophyton ''Trichophyton'' is a genus of fungi, which includes the parasitic varieties that cause tinea, including athlete's foot, ringworm, jock itch, and similar infections of the nail, beard, skin and scalp. Trichophyton fungi are molds characterized ...

'', ''

Epidermophyton ''Epidermophyton'' is a genus of fungus causing superficial and cutaneous mycoses, including ''E. floccosum'', and causes tinea corporis (ringworm), tinea cruris (jock itch), tinea pedis (athlete's foot), and tinea unguium Dermatophytosis ...

'', and ''

Microsporum ''Microsporum'' is a genus of fungi that causes tinea capitis, tinea corporis, ringworm, and other dermatophytoses (fungal infections of the skin). ''Microsporum'' forms both macroconidia (large asexual reproductive structures) and microconidia ...

''. Although dermatophytosis is fairly common fungal skin infection worldwide, it is more prevalent in areas with high humidity and environmental temperature. It is estimated that approximately 20-25% of world population are effected by superficial fungal infections, with dermatophytosis predominating. *

Oral candidiasis Oral candidiasis, also known as oral thrush among other names, is candidiasis that occurs in the mouth. That is, oral candidiasis is a mycosis (yeast/fungal infection) of ''Candida'' species on the mucous membranes of the mouth. ''Candida albica ...

, also commonly referred to as oral thrush, is a fungal infection caused mainly by ''

Candida albicans ''Candida albicans'' is an opportunistic pathogenic yeast that is a common member of the human gut flora. It can also survive outside the human body. It is detected in the gastrointestinal tract and mouth in 40–60% of healthy adults. It is us ...

'', which effects mucosal membranes of the oral cavity and the tongue. ''C. albicans'' accounts for approximately 95% of oral thrush cases. The fungus is part of the normal oral flora and only causes an infection when host immune and microbiota barriers are impaired, providing ''C. albicans'' with an opportunity to overgrow. It is estimated that oral candidiasis effects approximately 2 million people every year worldwide. *

Onychomycosis Onychomycosis, also known as tinea unguium, is a fungal infection of the nail. Symptoms may include white or yellow nail discoloration, thickening of the nail, and separation of the nail from the nail bed. Toenails or fingernails may be affected ...

, is a fungal infection which predominantly effects toenails. Two most common causative agents of onychomycosis are ''

Trichophyton mentagrophytes ''Trichophyton mentagrophytes'' is a species in the fungal genus ''Trichophyton''. It is one of three common fungi which cause ringworm in companion animals. It is also the second-most commonly isolated fungus causing tinea infections in humans, ...

'' and ''

Trichophyton rubrum ''Trichophyton rubrum'' is a dermatophytic fungus in the phylum Ascomycota. It is an exclusively clonal, anthropophilic saprotroph that colonizes the upper layers of dead skin, and is the most common cause of athlete's foot, fungal infectio ...

''. Common signs and symptoms include nail discolouration and thickening, nail separation from nail bed, and nail brittleness. Estimated prevalence of onychomycosis in North America is between 8.7% to 13.8%.

Parasitic

Parasitic infestations of the skin are caused by several

phyla Phyla, the plural of ''phylum'', may refer to: * Phylum, a biological taxon between Kingdom and Class * by analogy, in linguistics, a large division of possibly related languages, or a major language family which is not subordinate to another Phyl ...

of organisms, including Annelida,

Arthropoda 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 ...

Bryozoa Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic invertebrate animals, nearly all living in sedentary colonies. Typically about long, they have a special feeding structure called a l ...

Chordata A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These fiv ...

Cnidaria Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter. Their distinguishing feature is cnidocytes, specialized cells that th ...

Cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...

Echinodermata An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the sea li ...

Nemathelminthes The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-parasitic nematodes also known as eelworms. They are a diverse animal phylum inhabiting a bro ...

Platyhelminthes The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek language, Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a Phylum (biology), phylum of relati ...

, and

Protozoa Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris. Histo ...

Viral

Virus-related cutaneous conditions caused by these obligate

intracellular This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...

agents derive from both DNA and

RNA Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...

viruses. Some examples of viral skin infections include: *

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, are benign proliferative skin lesions that are caused by human papilloma virus (HPV). Warts vary in shape, size, appearance, and location on the body where they occur. For example,

plantar wart A plantar wart, or verruca vulgaris, is a wart occurring on the sole (foot), bottom of the foot or toes. Its color is typically similar to that of the skin. Small black dots often occur on the surface. One or more may occur in an area. They may r ...

s (verrucae plantaris), occur on the soles of the feet and appear as thick calluses. Other types of warts include

genital wart 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 ...

flat wart Flat warts, technically known as Verruca plana, are reddish-brown or flesh-colored, slightly raised, flat-surfaced, well-demarcated papule of 2 to 5 mm in diameter. Upon close inspection, these lesions have a surface that is "finely verrucou ...

s, mosaic warts, and

periungual wart Periungual warts are warts that cluster around the fingernail or toenail. They appear as thickened, fissured cauliflower-like skin around the nail plate. Periungual warts often cause loss of the cuticle and paronychia. Nail biting increases sus ...

s. Common treatment options include salicylic acid and cryotherapy with liquid nitrogen. *

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 ...

, is a highly contagious skin disease caused by the varicella-zoster virus (VZV). It is characterized by pruritic blister-like rash which may cover entire body, affecting all age groups. Rates of chickenpox are higher in countries which lack adequate immunization programs. In 2014, it has been estimated that global incidence of serious chickenpox infections requiring hospitalizations was 4.2 million. *

Hand, foot, and mouth disease Hand, foot, and mouth disease (HFMD) is a common infection caused by a group of enteroviruses. It typically begins with a fever and feeling generally unwell. This is followed a day or two later by flat discolored spots or bumps that may blis ...

(HFMD), is a common, often self-limiting viral illness which typically affects infants and children, however, it may also occur in adults. It is characterized by low grade fever and maculopapular rash on palms of the hands, soles of the feet, and around mouth. It is caused by the human

enterovirus ''Enterovirus'' is a genus of positive-sense single-stranded RNA virus An RNA virus is a virusother than a retrovirusthat has ribonucleic acid (RNA) as its genetic material. The nucleic acid is usually single-stranded RNA ( ssRNA) but it ma ...

es and

coxsackievirus Coxsackieviruses are a few related enteroviruses that belong to the ''Picornaviridae'' family of viral envelope, nonenveloped, linear, positive-sense single-stranded RNA viruses, as well as its genus ''Enterovirus'', which also includes poliovi ...

es, a positive-sense single-stranded RNA viruses.

References

{{Diseases of the skin and appendages by morphology Cutaneous conditions Infection-related cutaneous conditions