Psoriasis is a chronic, immune-mediated disease that causes inflammation and raised, scaly patches on the skin due to systemic inflammation. It occurs because the overactive immune system speeds up skin cell growth, causing skin cells to multiply up to 10 times faster than normal. Normal skin cells completely grow and shed in a month, but with psoriasis, skin cells do this in only three or four days, leading to a buildup of old cells being replaced by new ones that create the scales. Psoriasis symptoms vary from person to person, but some common ones include dry, itchy, raised skin patches covered with scales, most commonly on the knees, elbows, trunk, and scalp. Psoriasis can be painful, interfere with sleep, and make it hard to concentrate. The condition tends to go through cycles, flaring for a few weeks or months, then subsiding for a while. Psoriasis can also impact other organs and tissues in the body, and one in three people with psoriasis may also develop psoriatic arthritis. There is no cure for psoriasis, but treatment greatly reduces symptoms, even in serious cases.