Although the condition is most obvious on the skin (including the scalp, where it can cause dandruff), it's not really a skin disease. It's an autoimmune disease, meaning a person’s immune system has turned against the body. Because psoriasis comes from the immune system, it often affects other parts of the body as well. People with the condition have a higher risk for diabetes, stroke, and heart attacks. And about 30% of them get psoriatic arthritis, which causes swelling, stiffness, and pain in the joints.