Psoriasis causes red, pink, or purple patches of skin that are dry and scaly. The patches can appear anywhere on the body, but most often appear on the scalp, elbows, knees, or lower back. The severity of psoriasis can vary from small patches to covering the entire body. 

Psoriasis is caused by an overactive immune system that causes skin cells to multiply too quickly. The exact cause is unknown, but it's likely a combination of genetics and environmental factors. Family history and smoking can increase the risk of developing psoriasis.