Psoriasis is a chronic skin disease that is classically characterized by thickened, red areas of skin covered with silvery scales. The extent of skin involvement can range from discrete, localized areas to generalized body involvement. The joints, nails, scalp and mucous membranes may also be affected with the disease. Some cases of psoriasis are so mild that people don't know they have it. Severe psoriasis may cover large areas of the body. Psoriasis can be physically and emotionally disabling for those who suffer from it. Fortunately, there are new types of treatments available that work well, even for the most severe cases.