Skin infection

A skin infection occurs when germs invade the skin of humans or animals. These infections can also spread to associated soft tissues. These include mucous membranes and loose connective tissue. Experts often group these under the term skin and soft tissue infections, or SSTIs. They may also be called skin and skin structure infections, or SSSIs. Acute bacterial versions are known as ABSSIs. It is important to distinguish these from dermatitis. Dermatitis is simply inflammation of the skin. However, a skin infection can cause inflammation called infective dermatitis.

Bacteria are a major cause of these conditions. One common type is cellulitis, which is a diffuse inflammation of connective tissue. It causes severe inflammation in the dermal and subcutaneous layers. Cellulitis is divided into two main types based on symptoms. Purulent cellulitis is often caused by Staphylococcus aureus. This includes both methicillin-sensitive and methicillin-resistant strains, known as MRSA. Non-purulent cellulitis is usually linked to group A beta-hemolytic streptococci. One specific pathogen is Streptococcus pyogenes. In rare cases, this can progress into necrotizing fasciitis. This is a serious and potentially fatal infection.

Other bacterial infections affect different skin depths. Erysipelas is a bacterial infection that primarily targets the superficial dermis. It often involves the superficial lymphatics as well. Unlike cellulitis, erysipelas does not reach the deeper skin layers. It is mostly caused by Streptococcus pyogenes. Another condition is folliculitis, which affects the hair follicles in the dermal layer. While bacteria like Staphylococcus aureus are common causes, other agents exist. Fungi such as Malassezia species can cause it. Viruses like the herpes simplex virus are also possible. Even tiny mites, specifically Demodex species, can cause folliculitis.

Impetigo is a highly contagious form of acute bacterial skin infection. It is very common among children in preschool. This condition is primarily associated with S. aureus and S. pyogenes. It creates characteristic yellow, honey-colored crusted lesions. These lesions usually appear around the nose, mouth, and chin. It is estimated that 140 million people are affected by impetigo globally at any time. There are two main forms: bullous and nonbullous. Nonbullous impetigo is the most common, representing about 70% of cases. The bullous form accounts for the remaining 30%. Bullous impetigo is primarily caused by S. aureus. In rare instances, it can lead to Staphylococcal Scalded Skin Syndrome, or SSSS.

Fungi can also cause infections, sometimes called dermatomycoses. These can be superficial or deep in the skin, hair, or nails. Mycetoma is a broad group of fungal infections. These typically start in the subcutaneous tissues and skin of the foot. If not treated quickly, they can reach bones and joints. This can cause osteomyelitis, which is a bone infection. Severe cases might require surgical bone resections or limb amputation. As of 2010, mycetoma affected about one billion people globally. Another common fungal infection is dermatophytosis, or ringworm. This is a superficial infection caused by species like Trichophyton, Epidermophyton, or Microsporum. It is more common in areas with high temperature and humidity. It is estimated that 20% to 25% of the world's population is affected by superficial fungal infections.

Fungal infections also affect specific areas like the mouth and nails. Oral candidiasis, or oral thrush, affects the tongue and oral mucosal membranes. It is mainly caused by Candida albicans. This fungus is part of the normal oral flora. It only causes infection when microbiota or immune barriers are impaired. This allows the fungus to overgrow. About 2 million people are affected by oral candidiasis every year. Onychomycosis is a fungal infection that predominantly affects the toenails. Two common agents are Trichophyton mentagrophytes and Trichophyton rubrum. Symptoms include nail thickening, brittleness, and discoloration. In North America, prevalence is between 8.7% and 13.8%.

Parasites and viruses represent other ways the skin becomes infected. Parasitic infestations involve many different phyla, including Arthropoda and Protozoa. Viral infections are caused by obligate intracellular agents. These can be DNA or RNA viruses. Warts are benign proliferative skin lesions caused by the human papilloma virus, or HPV. Plantar warts, or verrucae plantaris, appear as thick calluses on the soles of the feet. Other types include flat, genital, mosaic, and periungual warts. Chickenpox is another highly contagious viral disease. It is caused by the varicella-zoster virus, or VZV. It causes a pruritic, blister-like rash. In 2014, an estimated 4.2 million serious cases required hospitalization. Finally, hand, foot, and mouth disease is caused by enteroviruses and coxsackieviruses. This viral illness often causes a rash on the palms, soles, and around the mouth.