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 speeds up, occurring in only a few days. This results in patches of thick, red skin covered with silvery scales. These patches (plaques) itch or feel sore. Plaque psoriasis is the most common form, but there are 4 other forms. These are:

• Guttate psoriasis. This leads to small, dot-like sores (lesions)
• Pustular psoriasis. This is marked by small, pus-filled bumps, weeping lesions, and intense scaling
• Inverse psoriasis. This causes very red areas in skin folds with little or no plaque
• Erythrodermic psoriasis. This is marked by intense shedding and redness.

Other common areas include other parts of the legs, low 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 3 in 10 of people with psoriasis develop inflammation of the joints and symptoms of arthritis. This is called psoriatic arthritis. Having psoriasis puts you at higher risk for diabetes, heart disease, stroke, cancer, Crohn's disease, liver 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. These include inflammation and faster turnover of skin cells. About one-third of cases of psoriasis are passed down through families. Strep throat can trigger the onset of guttate psoriasis, but psoriasis is not infectious.

Other possible triggers include:

• Damage to the skin in the form of scratches, vaccines, and sunburns
• Strep infections
• Dry weather
• Diet
• Allergies

Other medicines that may bring on an outbreak of psoriasis include the antidepressant lithium, medicines for malaria, some heart or blood pressure medicines, and some medicines for arthritis.

Exposure to the ultraviolet rays of the sun makes psoriasis better in many people. But sun exposure should be short so that the skin is not burned. Sunburns can trigger psoriasis. Hot packs don't relieve itching. Heat 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 yourself. The plaques may prevent you 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.

Healthcare providers often use a 1-2-3 approach in treating psoriasis. They start with topical treatment. Then they use light therapy, followed by medicine 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, healthcare providers may prescribe medicines that suppress the immune system. These include methotrexate, retinoids, cyclosporine, 6-thioguanine, hydroxyurea, and biologic response modifiers such as etanercept and adilumimab.