Cause: Psoriasis is thought to be caused by a faulty immune response as your body mistakes healthy cells for a threat. In fact, psoriasis is the most common autoimmune disease in the U.S., affecting as many as 7.5 million Americans, or more than 2 percent of all Americans. Psoriasis cannot be spread to others.
Where: This itchy, scaly rash occurs predominantly over joints including your elbows and knees and next to your scalp. In some instances, it can also affect your fingernails.
Learn more about psoriasis.