Psoriasis is a chronic skin disease in which itchy, scaly pink patches, usually covered with whitish scales, form on the elbows, knees, scalp and other parts of the body. It is one of the most common skin diseases in Britain, affecting about 2% of the population, but its cause is not known. There does seem to be some hereditary link in psoriasis, but often symptoms may not appear until adulthood. The severity of the condition can vary enormously from person to person and time to time. While psychological stress may cause an exacerbation of psoriasis, the only significant event that precipitates the disease is a preceding streptococcal infection. .