Psoriasis is a persistent skin disease that got its name from the Greek word for "itch." The skin becomes inflamed, producing red, thickened areas with silvery scales, most often on the scalp, elbows, knees, and lower back. Psoriasis is a chronic skin disease that generally appears as patches of raised red skin covered by a flaky white buildup. Psoriasis is a chronic (long-lasting) skin disease characterized by scaling and inflammation. Scaling occurs when cells in the outer layer of the skin reproduce faster than normal and pile up on the skin's surface. Although the exact cause is unknown, psoriasis is believed to be related to faulty signals sent by the body's immune system. These signals accelerate the growth cycle in skin cells, which pile up on the surface when the body can't shed them fast enough.

Psoriasis affects about 2 percent of the United States population, or about 7 million people. Although the disease occurs in all age groups and about equally in men and women, it primarily affects adults. People with psoriasis may suffer discomfort, including pain and itching, restricted motion in their joints, and emotional distress.

Psoriasis is not contagious-no one can "catch" it from another person. It has a genetic component that makes certain people more likely to develop it, but often an external or environmental "trigger" is necessary to make psoriasis appear. These triggers may include emotional stress, injury to the skin, some types of infection and reaction to certain drugs.

In its most typical form, psoriasis results in patches of thick, red skin covered with silvery scales. These patches, which are sometimes referred to as plaques, usually itch and may burn. The skin at the joints may crack. Psoriasis most often occurs on the elbows, knees, scalp, lower back, face, palms, and soles of the feet but it can affect any skin site. The disease may also affect the fingernails, the toenails, and the soft tissues inside the mouth and genitalia. About 15 percent of people with psoriasis have joint inflammation that produces arthritis symptoms. This condition is called psoriatic arthritis.