Apart from bacteriophage, which can readily be demonstrated in sebaceous material expressed from glands on the face, viruses cannot be considered a component of the normal flora of the skin. They are rarely, if ever, detected on the surface of the skin in the absence of clinically apparent lesions. Virus may be present in the deeper layers of the skin during the later incubation period after infection, or there may be latent infection with viruses such as papillomavirus. This is unlike the mucous membranes of the oropharynx, genitalia and gastrointestinal tract, where infective viruses may often be detected in the absence of clinical disease. For instance, herpes simplex virus (HSV) cannot be isolated from normal skin, although it may be found in the oropharynx of about 1 per cent of normal healthy adults; early studies suggesting its presence in uncomplicated eczemas have been contradicted.

The skin is a frequent site of manifestation of virus infection. Lesions may be localized or widespread as part of a systemic infection.

Microbiology

All viruses are obligate intracellular parasites and are characterized by containing only one type of nucleic acid, either DNA or RNA, which may be double or single stranded. These provide the main criteria for the classification of viruses. The virion comprises the nucleocapsid, which may be contained within an envelope derived from the cell membrane of the cell in which it replicated.