The prevalence of psoriasis varies from 0.1 to 11.8% in different populations. The highest prevalence in Europe is in Denmark (2.9%), and the average prevalence in Northern Europe is about 2%. The incidence in the United States ranges from 2.2 to 2.6%, with nearly 150,000 new cases diagnosed each year. East Africa has a higher incidence rate relative to West Africa, while Asia has the lowest incidence rate of about 0.4%. In a study of 26,000 Indians in South America, none were found to be affected. There was no significant difference in the incidence of psoriasis between males and females. Psoriasis can occur at any age, but it rarely occurs under 10 years of age. It usually first occurs between the ages of 15 and 30. Patients with specific HLA class I antigens, especially HLA-Cw6, usually have a young age of onset and a positive family history. This finding led Henseler and Christophers to suggest that two distinct categories of psoriasis exist: type I psoriasis, with a first age of less than 40 years and HLA association. type II psoriasis, with a first age of greater than 40 years and no HLA association, although many patients do not fit this classification. There is no evidence that type I and type II psoriasis respond differently to different treatments.