Psoriasis is a chronic skin condition that causes raised red or white patches topped with silvery, scaling skin. The patches are most common on the knees, elbows, scalp, tailbone, and back, but they may be anywhere on the body (including the fingernails, palms, and soles of the feet).

The patches (called plaques) are made up of dead skin cells that form thick layers. The body replaces normal skin cells every 28 days, but in psoriasis, skin cells are replaced every 3 to 6 days.

Psoriasis is not contagious. The exact cause of psoriasis is not known, but the body's natural defense system (immune system) is believed to be involved. The condition tends to run in families.

Small patches of psoriasis can often be treated with regular use of prescription steroid creams. Tar products (lotions, gels, shampoos) may also be useful, although they may increase sensitivity to the sun. Limited exposure to the sun may also help; unaffected skin should be protected with sunscreen. If psoriasis affects the scalp, dandruff shampoo may help. Severe psoriasis can be treated with stronger prescription medications and/or ultraviolet (UV) light therapy.