Skin Infection

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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 disease

Superficial fungal infections of the skin are the most common type of pathogenic fungal infection. 

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 streptococci

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 syndrome).

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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