Psoriasis is a disease whose main symptom is gray or silvery flaky patches on the skin which are red and inflamed underneath. In the United States, it affects 2 to 2.6 percent of the population, or between 5.8 and 7.5 million people. Commonly affected areas include the scalp, elbows, knees, navel, palms, ears and groin. Psoriasis is autoimmune in origin, and is not contagious.