Psoriasis is a common skin problem. It occurs equally in men and women, at any age, and tends to come and go unpredictably. It is not infectious, and does not leave any scar on the skin. 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 event, such as a throat infection, stress or an injury to the skin, or a dry weather.