Psoriasis is a chronic, inflammatory, and hyperproliferative skin disease associated with the immune system and genetics. The prevalence of the disease varies depending on the study methodology, type of measure, study size, age group, case definition, and sampling techniques. Globally, psoriasis affects approximately 0.91% to 8.5% of adults and 0% to 2.1% of children. Among the risk factors, smoking and obesity have a more negative impact than alcohol and tobacco use, diet, physical inactivity, infections, drugs, and mental disorders. Psoriasis can occur at any age, and when it appears before age 30, it is defined as type I. There is a familial aggregation of the disease; the HLA-Cw6, B57, and B31 alleles and HLA-Cw2 have been observed in patients with psoriasis type I.