Psoriasis is a chronic, recurring disease that causes one or
more raised, red patches that have silvery scales and a distinct
border between the patch and normal skin. The patches of
psoriasis occur because of an abnormally high rate of growth of
skin cells. The reason for the rapid cell growth is unknown, but
a problem with the immune system is thought to play a role. The
disorder often runs in families. Psoriasis is common and affects
about 1 to 5% of the population worldwide. Light-skinned people
are at greater risk, whereas blacks are less likely to get the
disease. Psoriasis begins most often in people aged 10 to 40,
although people in all age groups are susceptible. It usually
starts as one or more small patches on the scalp, elbows, knees,
back, or buttocks. The first patches may clear up after a few
months or remain, sometimes growing together to form larger
patches. Some people never have more than one or two small
patches, and others have patches covering large areas of the
body. Thick patches or pa