Psoriasis is a skin condition in which red scaly patches develop and show on the skin. These areas are most common on the scalp, elbows and knees, but they can occur on any area of the body. When the patches are on the scalp and in body fold areas, the skin is often itchy, but many people with psoriasis do not feel itchy at all.

The main problems caused by these patches are by shedding scales and by showing on the skin where they can be seen. Fortunately, psoriasis is unlikely to affect the face, and usually occurs on areas covered by clothes.

It is known that the skin in psoriasis patches is growing much quicker than normal skin. In all of us, the epidermis (the outer layer of the skin) grows continuously from its outer surface and a new layer is reformed each month. In psoriasis, the skin forms a complete layer of epidermis each 3-4 days - so that extra skin must be shed in scales. This process is similar to healing of the epidermis after an injury - except that normal skin 'knows when to slow down and psoriasis skin does not'. However the actual chemical cause of psoriasis is not known. There have been many scientific experiments performed looking for the cause, but so far the changes found seem to result form the rapid growth, and not to cause it.

Sometimes (but not certainly always) psoriasis can run in families, and so it is likely that there are inherited properties causing a "tendency" to the condition. If a person has this tendency, an accident or nervous shock, or some germ infections, can set off the condition and make it appear on the skin.