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.