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 process 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. There is also inflammation and blood vessel proliferation on the affected skin; the reasons for these are still not fully understood. There is activation of the immune cells in the skin causing inflammation. 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. 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 (e.g. hydroxychloroquine), and other drugs such as beta blockers (medicines commonly used to treat angina and high blood pressure). There might be a relationship between diet and psoriasis, but is still not fully understood, in general we advise avoiding pro inflammatory foods, alcohol and caffeine