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The earliest descriptions of what appears to represent psoriasis are given at the beginning of medicine in the Corpus Hippocraticum. This work was edited in Alexandria 100 years after the death of Hippocrates (460–377 bc), who presumably was the author. Hippocrates used the terms psora and lepra for conditions that can be recognized as psoriasis. Later, Celsus (ca. 25 bc) described a form of impetigo that was interpreted by R. Willan (1757–1812) as being psoriasis. Willan separated two diseases as psoriasiform entities, a discoid lepra Graecorum and a polycyclic confluent psora leprosa, which later was called psoriasis. In 1841, the Viennese dermatologist Ferdinand von Hebra (1816–1880) unequivocally showed that Willan's lepra Graecorum and psora leprosa were one disease that had caused much confusion because of differences in the size, distribution, growth, and involution of lesions.
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Psoriasis is universal in occurrence. However, its prevalence in different populations varies from 0.1% to 11.8%, according to published reports.1 The highest reported incidences in Europe have been in Denmark (2.9%) and the Faeroe Islands (2.8%). A recent study of 1.3 million Germans found a prevalence of 2.5%.2 Similar prevalence (ranging from 2.2% to 2.6%) has been measured in the United States. A higher prevalence in East Africans as opposed to West Africans may explain the relatively low prevalence of psoriasis in African-Americans (1.3% vs. 2.5% in white Americans).3 The incidence of psoriasis is also low in Asians (0.4%), and in an examination of 26,000 South American Indians, not a single case was seen. Psoriasis is equally common in males and females.4,5
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Psoriasis may begin at any age, but it is uncommon under the age of 10 years. It is most likely to appear between the ages of 15 and 30 years. Possession of certain HLA Class I antigens, particularly HLA-Cw6, is associated with an earlier age of onset and with a positive family history. This finding led Henseler and Christophers6 to propose that two different forms of psoriasis exist: type I psoriasis, with age of onset before 40 years and HLA-associated, and type II, with age of onset ...