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The normal turnover of skin cells—old cells dying and falling off, new cells rising to the surface—takes about a month. With psoriasis, the process is accelerated, occurring in only a few days. This results in patches of thick, red skin covered with silvery scales. These patches (also called plaques)itch or feel sore. Plaque psoriasis is the most common form, but there are four other forms. These include guttate, which leads to small, dot-like lesions; pustular, which is marked by weeping lesions and intense scaling; inverse psoriasis, which causes very red areas in skin folds with little or no plaque; and erythrodermic, which is marked by intense shedding and redness.

Other common areas include other parts of the legs, lower back, fingernails, toenails, face, and soles of the feet. Psoriasis also may affect the soft tissues of the genitals and inside the mouth.

Up to 30 percent of people with psoriasis develop inflammation of the joints and symptoms of arthritis. This is called psoriatic arthritis. According to the NPF, psoriasis is associated with increased risk of developing diabetes, heart disease, and depression.

Psoriasis occurs when white blood cells that normally protect the body against disease are triggered by mistake. These white blood cells, called T cells, cause other immune responses in the body, including inflammation and accelerated turnover of skin cells. About a third of cases of psoriasis are passed down through families.

The NPF also lists these as possible triggers: damage to the skin in the form of scratches, vaccinations, and sunburns; strep infections; dry weather; diet; and allergies. Other medications that may bring on an outbreak of psoriasis include the antidepressant lithium, medicines for malaria, some heart medications, and some medications for arthritis.

Exposure to the ultraviolet rays of the sun improves psoriasis in many people, but sun exposure should be short so that the skin is not burned. Sunburns can trigger psoriasis. Hot packs do not relieve itching; warmth tends to make the itching worse. Another suggestion to relieve itching: Wear light clothing that won't scratch your skin.

Itching may also interfere with walking or taking care of one's self. The plaques may prevent a person from playing sports or caring for family members. Medical care for psoriasis may interfere with work or school. The distress of having psoriasis may lead to depression and social isolation.

Doctors often use a 1-2-3 approach in treating psoriasis: topical treatment, followed by light therapy, followed by medication that treats the entire immune system. Topical treatments include corticosteroids or anthralin to reduce inflammation and turnover of skin cells; calcipotriene, a synthetic form of vitamin D; retinoids, a form of vitamin A; coal tar; salicylic acid; clobetasol propionate; bath oil; and moisturizers. Phototherapy, in the form of sunlight or an ultraviolet-B light box, causes the activated T cells to die. This eases the inflammation and slows the turnover of skin cells. Light therapy also may include the topical medicine psoralen combined with ultraviolet A rays; this combo is called PUVA. For more severe forms of psoriasis, doctors may prescribe medicines that suppress the immune system. These include methotrexate, retinoids, cyclosporine, 6-thioguanine, hydroxyurea, and biologic response modifiers.