Psoriasis is a common skin problem affecting about 2% of the population. It occurs
equally in men and women, at any age, and tends to come and go unpredictably. It
is not infectious, and does not scar the skin.
The skin is a complex organ made up of several different layers. The outer layer
of skin (the epidermis) contains cells which are formed at the bottom and then move
up towards the surface, gradually changing as they go, finally dying before they
are shed from the surface. This journey normally takes between 3 and 4 weeks. In
psoriasis, the rate of turnover is dramatically increased within the affected skin,
so that cells are formed and shed in as little as 3 or 4 days. The reasons for this
are still not fully understood.
Some people are more likely to develop psoriasis than others, particularly if there
is someone else in their family who has psoriasis: in other words, it is a genetic
or hereditary disease (see below). However, the trigger for psoriasis to appear is
often an outside event, such as a throat infection, stress or an injury to the skin.
In practice, for most patients who develop psoriasis, or for whom it clears and then
comes back, no obvious cause can be detected. Usually, sunlight improves psoriasis,
though occasionally it makes it worse (especially if the skin gets burned). A high
alcohol intake and smoking can worsen psoriasis too, as can medicines used for other
conditions - such as lithium, some tablets used to treat malaria, and other drugs
such as beta blockers (medicines commonly used to treat angina and high blood pressure).
There is no apparent relationship between diet and psoriasis.