This chronic, relapsing inflammatory skin disease affects about 2 per cent of the UK population. Frequently it is mild or trivial, affecting only the points of the elbows or knees and the scalp, but in a substantial minority of sufferers the disease is much more widespread and causes considerable discomfort and social embarrassment. Rarely, it can even be life-threatening.

The predisposition to psoriasis is genetic, multiple genes being involved, but postnatal factors such as acute infection, hormonal disturbance, pregnancy and drugs can influence or provoke it.

The psoriatic lesion is dull red, scaly and well defined. Scale is shed constantly, either in tiny pieces or as large plaques. The scalp is usually affected but the disease does not cause significant hair loss. The fingernails may be pitted or ridged and the toenails grossly thickened. Several clinical patterns occur: in guttate psoriasis, a sudden explosion of multiple tiny lesions may follow a streptococcal throat infection, especially in children. Larger lesions are characteristic of discoid (plaque) psoriasis, the usual adult form. In the elderly the plaques may be mainly in the large body folds – flexural psoriasis. Rarely, psoriasis may be universal (psoriatic erythroderma), or a sterile pustular eruption may supervene (pustular psoriasis).

Psoriasis does not affect internal organs, but in about 1 per cent of sufferers an inflammatory joint disease (psoriatic arthritis) may be associated with the condition.