The condition is, at least partly, genetically based. About a third of the close relatives of someone with psoriasis will also have the disease, and in identical twins, about 70 percent will both have the disease if one of them has it. It appears to involve an immune stimulation, which leads to an overgrowth of skin cells that fail to mature completely. The disease is not infectious or sexually transmitted. It cannot be passed between husbands and wives.
Psoriasis can appear at any age, but becomes less common in older people.
It tends to appear at a later age in men than in women. The younger someone is when it appears, the more severe the condition usually proves to be. The typical lesion of psoriasis is a red, slightly raised plaque with fine silvery scales on it which can be flaked off. People with extensive plaques, especially on the scalp, can have an enormous amount of flaking. The plaques are found most commonly on the elbows, knees, scalp, and lower back, but there are many variations, such as plaques around the buttocks, anus, and genitalia or involving the soles of the feet and palms of the hands.