Psoriasis is a genetic and hereditary disease, so you may have it because one of your forebears or ancestors had or carried it.
In other words, they may not have suffered from it but it was in their genetic make-up, just like so many other diseases.
So to explain in a little more detail, for example, more people have a predisposition to say cancer, or diabetes, or pretty much anything else, than suffer from these diseases.
The reasons why one person will become a sufferer of a disease and another simply a carrier are not always fully understood, but it often involves something that "triggers" the disease to become active, and this is what is thought to happen in the case of psoriasis.
After all, we all know seemingly healthy people who have suffered from diseases you wouldn't have thought they would have done.
In the case of psoriasis, scientists believe that at least 10% of the general population have a predisposition to psoriasis.
However, only 2% to 3% of the population develops the disease.
This is thought to be because only 2% to 3% of people encounter the "right" mix of genetics and triggers that lead to the development of psoriasis.
Those who have a genetic disease, but don't have an obvious family history of it, may for example have "inherited" say two genes from their father and say two from their mother-neither of whom had all four (not that 4 are necessarily required) and therefore never developed the disease.
Or perhaps they never had the disease "triggered" by the right, or depending on how you look at it, wrong, triggers.
Psoriasis is found worldwide but its frequency varies among different ethnic groups and regions of the world.
If I said to you that it is estimated that around 125 million people, or roughly 1.
75% of the world's population, suffer from psoriasis, that might help give you some indication as to the sheer scale of the "why me".
Of course, the 125 million figure is an estimate.
It is quite possible that the number is significantly higher.
Put another way, it is the second most common skin disease worldwide, the first being acne.
It terms of large geographical areas, it is commonest in Scandinavia and northern Europe, where it approaches 3%, whereas in the UK the prevalence is around 2%.
Around 2% of North Americans have it.
In fact, psoriasis is the most prevalent autoimmune disease in the U.
S.
In Native American Indians and aboriginal Australians it is almost non-existent.
In Japan it is 0.
2%.
In a study of twin pairs in Australia, psoriasis occurred more frequently in southern states when compared with the warmer northern states.
The same type of geographic variation was noted in a study in Norway, which showed higher rates in the northern and cooler parts of the country and lower rates in the southern regions relative to the rest of the country.
Further research shows that psoriasis amongst African Americans at 1.
3% is statistically significantly lower than that in white Americans at 2.
5%.
The onset of psoriasis can occur at any age from birth through to advanced years.
However, many large studies show that the age of onset of psoriasis appears to peak in two main periods.
These are in early adult life (late teens to 20s) and then again in late adult life (50s and 60s).
It is thought that these different age peaks represent two different types of psoriasis, so-called type I and type II.
Type I is believed to occur before the age of 40 and thought to account for more than 75% of cases.
Patients with this type of psoriasis tend to be more severely afflicted and have more relatives affected than the type II.
Type II is therefore the later in life occurring form of psoriasis affecting a minority of sufferers.