This is an interesting article about how old the genes are that play a role in Crohn’s and psoriasis.
“Both diseases are autoimmune disorders, and one can imagine that in a pathogen-rich environment, a highly active immune system may actually be a good thing even if it increases the chances of an auto-immune response.”
The question they don’t ask, and we’ll never know, is if these ancient ancestors suffered from the symptoms of these issues. Then you could ask the question, like in the book “The Epidemic of Absence”, would people who suffer from Crohn’s today, benefit from from being exposed to pathogens?
http://www.sciencedaily.com/releases/2015/01/150128114058.htm