According to the National Psoriasis Foundation, psoriasis affects 7.5 million Americans and is one of the most common autoimmune diseases in the United States. Psoriasis is a chronic (long lasting) disease that presents on the skin and in the joints. It develops when a person’s immune system sends faulty signals that direct skin cells to grow too quickly, causing them to form in days rather than in weeks. The body does not shed these excess skin cells so they pile up on the surface of the skin, resulting in patchy skin, scaling and plaques. Although psoriasis may occur on any skin surface, it is often seen on the elbows, knees and the scalp. Psoriasis can affect joints and cause nail changes, including nail thickening, yellow-brown spots, dents (pits) on the nail surface and separation of the nail from the base.