Psoriasis is a chronic condition with no known cure. It’s caused by improper functioning of your immune system. The condition makes new skin cells develop unnecessarily on top of your existing, healthy skin. The resulting patches can appear anywhere on the body, but most often affect the skin at the elbows, knees, scalp, fingernails, and toenails. Psoriasis can also generate joint inflammation, which is known as psoriatic arthritis.