Psoriasis is a chronic, recurrent, inflammatory disorder that can affect the skin, nails, and joints. The most common form of psoriasis is plaque, or discoid, psoriasis, which has localised, raised, and inflamed silvery-scaled lesions that are most frequently present on the surface of the knees, elbows, and scalp. It affects about two per cent of the population in Europe and North America, but is less common in other regions. In terms of causes, there may be a genetic tendency. The basic problem with psoriasis is that new skin cells form more quickly than usual, causing accumulations of living cells. These create thickened patches of skin covered with dead, flaking cells. The condition will vary in severity from person to person and can, sometimes, lead to infection.