Thick, scaly, red plaques are the hallmark of psoriasis. In psoriatic skin, the cells of the outer layer (epidermis) multiply too rapidly, which causes skin to thicken. They also adhere to one another more strongly and for longer than normal skin cells do, resulting in scaliness. The skin is infiltrated by white blood cells, causing inflammation, redness, and rarely pustules. Why this happens is not well known, but genetics are clearly involved.
People of any age can get psoriasis. It can be detected on the skin of infants before they're even born or can appear for the first time in those well into their senior years. Most patients first get the disease either in early adult life or when they're in their fifties. Some with psoriasis have the disease throughout their lifetime, although the severity can range from barely noticeable to hard to ignore. Certain factors can worsen psoriasis, including medications and skin trauma.
Psoriasis is not infectious and cannot be passed directly from one person to another. Although very rapid growth of skin cells is seen in psoriasis, it is still a harmless process and not related to cancer.