INFECTIONS OF THE SKIN
The skin is a major element of the innate immune defense . It is normally colonized by a variety of microorganisms although the numbers and types vary between different areas of the body. In normal circumstances it forms an effective barrier to invading microorganisms.
Papillomaviruses can infect epidermal cells and stimulate their proliferation to form warts. Numerous herpes viruses have been described and at least eight of them, the Human Herpes Viruses (HHV1-8), infect humans and can cause clinical disease. Largely due to historical reasons, they are also known by other names (Table 3.1). For example, Herpes simplex viruses 1 and 2 may infect skin of the genitalia causing Herpes labialis and Herpes genitalis respectively and Varicella zoster causes chickenpox and shingles.
Bacterial skin infections normally occur only if the normal balance between the skin environment and these organisms is disturbed. The outbreaks of acne caused by Propionibacterium species (Figure 3.1) during the hormonal changes associated with puberty are typical. Breaks in the skin from wounds or surgery or lesions from insect bites or chickenpox may also lead to infections of the skin by Staphylococcus aureus (Figure 3.2) and Streptococcus pyogenes. In children especially, they may cause impetigo contagiosa, an extremely contagious skin infection. Staphylococcus aureus can colonize hair follicles leading to inflammations that can develop into abscesses (boils) or even, in extreme cases, carbuncles: an amalgam of several abscesses. Enterobacteria,
Pseudomonas aeruginosa, Staphylococcus aureus and Streptococcus pyogenes, are all associated with a variety of skin problems following weakening of the patient, for example, by diabetes , by a deficiency of the immune system , by nosocomial, or hospital acquired, infection of surgical wounds or burning of the skin. Mycobacterium leprae is the causative organism of the dreaded disease leprosy, which, although now virtually eliminated in developed countries, still affects many thousands in the developing world.
Fungi are associated with a variety of infections, particularly of the skin.