Because the skin is in contact with the environment, it is especially susceptible to injuries, such as abrasions (scraping), contusions (bruises), and cuts. Other common disorders of the skin may be subdivided into infectious and noninfectious disorders. Some inflammatory disorders may fall into either group, depending on the specific cause of the disorder. Common childhood diseases, such as chicken pox and measles, are not listed here but produce skin lesions that characterise the particular disease.

Infectious Disorders

Acne (ak’-ne) is a chronic skin disorder characterised by plugged hair follicles that often form pimples (pustules) due to infection by certain bacteria. It often appears at puberty, when sex hormones stimulate increased sebum secretion.

Athlete’s foot (tinea pedis) is a slightly contagious infection that is caused by a fungus growing on the skin. It produces reddish, flaky, and itchy patches of skin, especially between and under the toes, where moisture persists.

Boils are acute, painful Staphylococcus infections of hair follicles and their sebaceous glands as well as the surrounding dermis and subcutaneous tissue. The union of several boils forms a carbuncle.

Fever blisters, or cold sores, are clusters of fluid- filled vesicles that occur on the lips or oral membranes. They are caused by a Herpes simplex virus (type 1) and are transmitted by oral or respiratory exposure. Genital herpes, which is caused by either Herpes simplex virus type 1 or Herpes simplex virus type 2, results in the formation of painful blisters on the genitals as a result of infection transmitted by sexual activity.

Impetigo (im-pe-ti’-go) is a highly contagious skin infection caused by bacteria. It typically occurs in children and is characterised by fluid-filled pustules that rupture, forming a yellow crust over the infected area.

Non-infectious Disorders

Alopecia (al-o-pay’-she -ah) is the loss of hair. It is most common in males who have inherited male pattern baldness, but it may result from noninherited causes, such as poor nutrition, sensitivity to drugs, and eczema.

Bed bugs (Cimex lectulariur) are microscopic parasitic insects that feed almost exclusively off human blood. Their preferred habitats are sleeping areas in hotels and homes, but they can also be found in office buildings, movie theatres, and public transportation vehicles. Bed bugs exhibit peak feeding activity at night, with a preference for exposed areas of skin, and leave behind itchy welts. Washing infested clothes and bedding at 115° F (46°C) will kill bed bugs. Insecticides, deep cleaning of infested areas, and discarding mattresses are recommended for living areas and other contaminated areas.

Bedsores (decubitus ulcers) result from a chronic deficiency of blood circulation in the dermis and subcutaneous tissue. Bedsores form over bones that are subjected to prolonged pressure against a bed or cast. They are most common in bedridden patients. Frequent turning of bedridden patients helps to prevent bedsores.

Blisters, fluid-filled pockets, form when an abrasion, burn or injury causes the epidermis to separate from the dermis.

Burns are damage to the skin caused by heat, chemicals, or radiation. Burns are classified according to the degree of damage. A first-degree burn involves only the epidermis. Healing is usually rapid. A second-degree burn produces damage to the epidermis and the superficial portion of the dermis. Painful blisters form between the epidermis and dermis but usually, no infection occurs. Healing requires two to several weeks, and scarring may occur. First and second-degree burns are also referred to as partial thickness burns because they do not penetrate through the full thickness of the skin. A third-degree burn (or full thickness burn) destroys the epidermis, dermis, glands, hair follicles, and nerve endings. Most cases of third-degree burns are painless due to the complete destruction of nerve endings. Normal skin functions are lost, so care must be given in order to control fluid loss and bacterial infection. Skin grafting is often necessary, and scarring usually results.

Calluses and corns are thickened areas of skin that result from chronic pressure. Calluses are larger and often occur on the palms and on the balls of the feet. Corns are smaller and usually occur on the superior surface of the toes. Improperly fitting shoes are frequent causes of corns on the feet.

A common mole (nevus) is a pink, tan, or brown growth usually appearing in childhood and continuing to develop into adulthood. Common moles result when melanocytes grow in clusters and rarely do they develop into melanoma.

Dandruff (seborrheic dermatitis) is the excessive shedding of dead epidermal cells from the scalp as a result of excessive cell production. It is usually caused by seborrheic eczema of the scalp, noninfectious dermatitis.

Eczema (ek-ze ‘-mah) (atopic dermatitis) is an inflammation-producing redness, itching, scaling, and sometimes cracking of the skin. It is noninfectious and noncontagious, and it may result from exposure to irritants or from allergic reactions. Seborrheic eczema is characterised by hyperactivity of sebaceous glands and patches of red, scaling, and itching skin. It may occur at the corners of the mouth, in hairy areas, or in skin exposed to irritants.

Hives are red, itchy bumps or wheals that usually result from an allergic reaction to certain foods, drugs, or pollens.

Psoriasis (so-ri’-ah-sis) is a chronic, noncontagious dermatitis that is characterised by reddish, raised patches of skin that are covered with whitish scales. It results from excessive cell production that may be triggered by emotional stress or poor health. The affected area may be slightly sore or may itch. Psoriasis occurs most often on the scalp, elbows, knees, buttocks, and lumbar areas.