The exact cause of psoriasis is unknown. What is known is the disease is not contagious. People cannot "catch" it by touching the skin of someone who has psoriasis. According to the National Psoriasis Foundation, psoriasis tends to run in families; scientists are exploring genetic links to the disease. The condition commonly appears between the ages of 15 - 35, and often becomes a life-long, chronic illness. Patients usually experience alternating periods of flare-ups and remissions. Psoriasis is known as an "equal opportunity" disease — it affects people of every socioeconomic class.

Recent discoveries lead scientists to suspect that psoriasis stems from an immune system disorder. An abnormality in the functioning of key white blood cell lymphocytes appears to trigger inflammation in the skin. Scientists are attempting to understand exactly what sets off the cascade of events that cause skin cells to divide too rapidly. This rapid shedding causes the thick skin and silvery plaques and scales associated with the disease.

Stress to the skin can trigger a flare-up of psoriasis. Patients have reported new plaques appearing about two weeks after the skin is cut, scratched, rubbed or severely sunburned. Psoriasis typically starts as small, red bumps that develop into larger scaly plaques. Infections, such as strep throat, can encourage a flare-up, as can the use of certain medications such as lithium. Emotional stress has been linked to outbreaks of psoriasis. Climate can also be a contributing factor; many patients say their psoriasis is worse in the winter, when their skin is dry and exposed to less sunlight.

Where it Occurs

Psoriasis commonly affects the elbows, knees, groin, buttocks, scalp and nails. Nails affected by psoriasis may become pitted or discolored. The nails may loosen, thicken or crumble. Inverse psoriasis shows up in the armpit, the skin under the breast, and in skin folds around the groin, buttocks and genitals. Guttate psoriasis affects children and young adults. It often occurs after a sore throat and may clear up in weeks or a few months. Pustular psoriasis may be widespread or localized with puss-filled blisters that form on the skin, and then dry and peel. Erythrodermic psoriasis is the rarest form, but can affect most of the skin and causes tremendous discomfort.

Psoriasis often appears in the same place on both sides of the body. About 10% of all cases are described as extensive, involving more than 20% of the body's surface area. Mild to moderate psoriasis affects less of the body's surface area. (One percent of the body surface area is usually the size of the palm of the hand.)

Approximately 10-30% of those with psoriasis also develop arthritis. Similar immune cells affect inflammation in both skin and joints. Some patients say their arthritis improves when the condition of their skin gets better and vice versa.

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