Psoriasis is a chronic skin disorder affecting more than 4 million people, producing silvery, scaly plaques on the skin. Psoriasis usually starts in adolescence or after age 60, affecting 2 percent of the population. The condition is mild if less than 10 percent of the body is affected, while more than 30 percent is considered severe. The location of the symptoms, more than the extent, influences how disabling the condition may be. Cycles of improvement and flare-ups are experienced, and the disease can go into remission for periods ranging from one to 60 years. The cause is still unknown, although researchers believe some type of biochemical stimulus triggers the abnormal cell growth in the epidermis. While normal skin cells take a month to mature, patients with psoriasis have skin cells that over-multiply, forcing cells to move up to the top of the skin in only seven days. As the number of cells build up, the epidermis thickens and the extra cells pile up in raised, red and scaly lesions. The inflammation is caused by the buildup of blood needed to feed the rapidly dividing cells. There is no cure but there are treatments that can clear plaques or significantly improve the skin's appearance. Treatment is aimed at slowing the excessive cell division, resulting in remissions lasting up to a year. Topical medications can be used and regular sun exposure may help clear up a case because of the exposure to natural ultraviolet light (UVB). A laser can also be used to treat and temporarily clear localized psoriasis. Newer drug treatments are in development to target and block specific parts of the body's immune reaction that leads to diseases.