We are not exactly sure why someone develops psoriasis however, we do know that genetics plays an important role. Research has shown that the immune system is overactive and causes the skin cells to grow at an abnormally fast rate. Sometimes there is an obvious trigger such as an infection with group A streptococcus or a medication such as a beta blocker. In many cases, however, the trigger is unclear. You are more likely to develop psoriasis if family members have it too. Approximately 1 in 3 people have an affected family member. On average, psoriasis affects 2-3% of the population. It usually shows up between 15-35 years of age, but can develop at any time. Up to 15% of psoriasis starts during childhood.