Psoriasis is a chronic skin condition which causes areas of the skin to become thickened, red, and scaly. It affects men and women, across all ethnicities. It is more prevalent in adults than in children-peak ages of onset are 15-25 years and 50-60 years.

The cause of psoriasis is unclear, but involves an abnormal response of the immune system resulting in increased skin turnover and the formation of plaques. It has several subtypes and can affect different parts of the body. The commonest subtype, chronic plaque psoriasis, affects the elbows, knees, lower back and scalp.