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Infections of the skin may be due to viruses,
fungi or bacteria, viruses being the most common causative organisms.
Viruses can have both direct and indirect effects in skin infections
Three main groups of viruses affect the skin directly, producing lesions in
which the virus particles are present (often in large numbers), sometimes in the
form of intracellular inclusions.
Herpesvirus produces blistering lesions of the epidermis, with viral inclusions
in the epidermal cells. Examples include herpes simplex type 1 (non-genital
herpes), herpes simplex type 2 (genital herpes), and herpes zoster-varicella
(chicken pox and shingles).
Human papillomavirus produces proliferative epidermal lesions (viral warts), the
nature of which differs from site to site, varying from exophytic filiform
fronded lesions or small flat plaques (e.g. planar warts on the face) to deeply
inverted hyperkeratotic lesions (e.g. plantar warts on the sole of the foot). In
the skin of the perineum, human papillomavirus produces florid papillomatous
exophytic growths called condyloma acuminata. the virus may also infect the
genital and anal mucosa in both males and females, and is regarded as an
aetiological factor in the development of carcinoma at these sites.
Pox virus produces multiple, pale, domed lesions on the trunk and face of
children, adolescents and young adults (molluscum contagiosum). These are due to
localized nodular thickening of the epidermis, the epidermal cells being packed
with large viral inclusions.
Fungi are common commensals on the skin surface and may give rise to skin
Superficial fungal infections of the skin are the most common type of pathogenic
In the vast majority of cases the fungal hyphae or yeast forms reside in the
keratin layer on the skin surface, and are associated with a minimal
inflammatory response in the epidermis and upper dermis.
These infections are commonly seen in community practice, the nature of the
lesion being dependent on the type of causative organism and the particular area
of skin involved. Common diseases such as athlete's foot (caused by Trichophyton
and Epidermophyton) and ringworm (caused by Microsporum or Trichophyton rubrum)
are conditions most familiar to the general public.
Occasionally the fungi populate the hair follicles, in which cases the
inflammatory reaction may be more vigorous, resulting in a destructive
folliculitis. This is becoming more common, particularly in immunosuppressed
patients (e.g. those with AIDS).
Bacterial infections of the skin are usually due to staphylococci or
Both staphylococci and streptococci can produce a condition called impetigo,
which is highly contagious and spreads rapidly through populations, e.g. in
schools. Large epidermal blisters (bullae) develop, which may be filled either
with clear fluid or with neutrophil polymorphs to form a pus-containing blister
(pustule). In infants, some staphylococci produce a potent toxin that can lead
to extensive disruption of the epidermis, with widespread confluent blistering
and separation of the upper layers of the epidermis (staphylococcal scalded skin
Infection of hair follicles by bacteria, usually Staphylococcus aureus, produces
tiny pustules located in the necks of the follicles (superficial folliculitis).
Deep, severe follicle infection leads to an expanding collection of pus, which
destroys the follicle and extends into surrounding dermis to form a boil.
Erysipelas is a spreading acute inflammation of the deep dermis and upper
sub-cutis by streptococci, most commonly seen on the face Cellulitis is a
similar infection, but it involves the deep sub-cutis and sometimes underlying
fascia. Caused by a variety of organisms including staphylococci and
streptococci, it may lead to extensive necrosis of the deep tissues of the skin.
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