Psoriasis is a chronic disease in which skin cells grow too quickly in some areas and do not shed properly. Because of this, the excess cells build up on the skin causing scaly patches. Psoriasis is not contagious. Psoriasis is caused by genes that are inherited from a parent. There are several types of psoriasis including plaque, guttate (smaller scattered spots), inverse (located in skin folds, such as the underarms), pustular, and erythrodermic.