No one knows the exact cause of psoriasis, but experts believe that it’s a combination of things. Something wrong with the immune system causes inflammation, triggering new skin cells to form too quickly. Normally, the body replaces skin cells every 10 to 30 days. With psoriasis, new cells grow every 3 to 4 days. The buildup of old cells being replaced by new ones creates the silvery scales associated with psoriasis.