Do you have normal skin? Then be very thankful. Around one in 50 people in Britain have patches of skin that are thickened, flaky, red, and itchy.

They may even sting all the time and bleed when clothes rub against them. On top of that, if you have this affliction, you feel as if the rest of the world is looking at you and thinks you are infectious. Children have been excluded unfairly from swimming pools because of it.

Even kingdoms have been shaken by it. King Robert Bruce of Scotland had it. King Edward of England put it about that Robert’s skin disease was actually leprosy – a disease that would automatically rule him out from being King. So when Robert Bruce’s psoriasis was at its height, he had to hide it from his people. Hence the story about him being in a cave and watching a spider trying and trying again to make its web across the cave mouth.

Not that being in the dark would have helped him much. He might have been better off in the sunshine, according to some of today’s accepted wisdom about the disease.

In Britain, psoriasis usually starts in childhood, the age of onset being under ten for girls and between 15 and 19 for boys. We don’t know why this is. About a third of people with psoriasis have relatives with it, strongly suggesting that a tendency to developing it is inherited.

People start with normal skin, then some sort of accident or infection seems to trigger it. For some reason, the immune system reacts to the event by causing the cells that produce the skin to multiply at seven times their normal rate.

Our skin is continually growing and shedding its outer layer. It normally takes a new skin cell 28 days to travel from its ‘birthplace’ under the skin to the outer surface. It takes only four days for a psoriatic cell to do the same.

So the skin is thicker at areas affected by psoriasis, which explains the rough surface. The cells are also ‘stickier’ so that they don’t shed as easily as normal skin cells. Because of that, the skin takes on a silvery appearance, as the layer of dead cells builds up.

To add to this, underneath the patch of psoriasis, the body’s immune system is actively trying to correct the problem. It does that by bringing into the area ‘inflammatory’ white blood cells and other substances that produces the irritation and itch and even bleeding.

For an idea of the sort of discomfort this can cause, think of being stung by nettles. The irritation under a patch of psoriasis is a similar process – except that you can’t walk away from the ‘nettles’. Patches of psoriasis on the skin are bad enough, but it can also deform the nails and cause a badly flaking scalp. Naturally psoriasis sufferers are particularly sensitive to these problems: they need a lot of understanding and sympathy.

The two main treatments are skin creams containing tar products (to remove the excess thickness of skin) and steroids (to damp down the inflammation). In recent years creams containing substances related to vitamin A (retinoids) and to vitamin D have been used. Most people with psoriasis respond initially to one or a combination of these treatments, but they can have side-effects and eventually they may lose their beneficial effects.

So people with severe psoriasis or whose usual treatment has failed are offered light treatment (phototherapy). Sometimes this is given alone, as ‘PUVB’, using ultraviolet light in a particular waveband. Other people are offered PUVA, in which a ‘psoralen’ or drug that makes the psoriatic skin more sensitive to ultraviolet light, is given along with the light treatment.

They have to be supervised by a specialist skin clinic team, to make sure that doses are correct and that the skin is not damaged by the treatment.

There are times when even this treatment is not enough. The same fault in the immune system that produces psoriasis may also produce a form of arthritis: arthritis is very much commoner than usual among people with psoriasis.

People with both psoriasis and arthritis may be offered one of the newer anti-arthritic drugs, such as methotrexate.

Other treatments for severe psoriasis that is badly affecting a person’s quality of life include the transplant anti-rejection drug cyclosporin, which does work exceptionally well, but may cause kidney problems and high blood pressure. These serious side effects reverse after the drug is stopped.

Some people with psoriasis find the sun helps; others find it gets worse in the summer and better in the winter. The jury is still out on that question 700 years after the first clinical trial in the wilds of medieval Scotland.