Viruses, bacteria, fungi, yeasts and parasites cause skin infections. Depending on the cause, they may produce common signs on the skin, or signs specific to the disease may appear. There are many viral skin infections. The most commonly encountered skin infections are cellulitis, erysipelas and impetigo (pyoderma).

Human skin normally harbours many saprophytic microorganisms that live without causing disease, and a small number of pathogenic ones. These bacteria make up the skin flora. The skin flora is of two kinds: resident and transient flora. Bacterial density is higher in exposed areas than in other areas.

Infections of the skin can arise from bacteria, fungi or viruses. Physiologically, the skin forms a barrier layer against these biological agents. The human body flora is one of the natural resistance mechanisms and begins to form at birth; in other words, a newborn baby is sterile. Most members of the normal flora are bacteria. With these features, our skin is a line of defence that wraps our body and protects it from external factors. The skin has a protective role against many diseases that may be caused by bacteria, fungi and viruses. Skin infections are very common in the population.