Psoriasis is a common and chronic skin disease of unknown cause. It can begin at any age but most commonly begins in young adults. In most cases both heredity and environment appear to influence the development of psoriasis. Once psoriasis occurs in an individual, it usually persists indefinitely if not treated, although in mild cases it may resolve and then recur spontaneously in cycles. Psoriasis has little effect on general health and is primarily a cosmetic problem except in severe cases, where thickened psoriatic skin may make movement painful.
Psoriasis can appear in several different forms.
The most common type of psoriasis consists of sharply demarcated red patches with adherent white scales. These occur most frequently on the scalp, elbows, knees, and lower back, but can occur anywhere on the skin. Psoriasis often gets worse with physical or emotional stress or with injuries to the skin. Fingernails and toenails can be affected with small pits in the nails, separation of nails from nailbeds, or abnormal growth of nails. A less common form of psoriasis, however, several other skin diseases may be mistaken for psoriasis.