Psoriasis is a chronic autoimmune disease which is usually characterized by patches red, itchy, and scaly skin. There are five main types of psoriasis: plaque, guttate, inverse, pustular, and erythrodermic. Plaque psoriasis, also known as psoriasis vulgaris, makes up about 90% of cases and commonly affects the back of the forearms, shins, around the navel, and the scalp. Guttate psoriasis has drop-shaped lesions. Pustular psoriasis has small non-infectious blisters. Inverse psoriasis forms red patches in skin folds. Erythrodermic psoriasis occurs when the rash becomes very widespread, and can develop from any of the aforementioned types.