We don't know. It's probably a combo of genetic factors and bacteria. It's at the very least complicated and multifaceted, because you can't just take a culture or biopsy of inflamed skin and say "A ha! This is clearly different!"
All the most effective treatments try to turn off parts of the immune system, and even though have minor success with some patients going through multiple different immunosuppressants to find the right one, or even cocktail, that adequately manages the disease.
All the most effective treatments try to turn off parts of the immune system, and even though have minor success with some patients going through multiple different immunosuppressants to find the right one, or even cocktail, that adequately manages the disease.