Parasites and autoimmune disease
http://www.newscientist.com/article.ns?id=dn10964&feedId=online-news_rss20
"One hypothesis that is gaining growing acceptance is that certain bacteria and parasites – which have lived alongside humans for millennia – have gradually developed ways of stimulating Treg cells to dampen down the immune system, enabling them to survive in the body for longer. With modern standards of hygiene these 'old friends' are gone and Treg cells are consequently less active, meaning the TH1 and TH2 cells go into overdrive, triggering allergies and autoimmune diseases."
"One hypothesis that is gaining growing acceptance is that certain bacteria and parasites – which have lived alongside humans for millennia – have gradually developed ways of stimulating Treg cells to dampen down the immune system, enabling them to survive in the body for longer. With modern standards of hygiene these 'old friends' are gone and Treg cells are consequently less active, meaning the TH1 and TH2 cells go into overdrive, triggering allergies and autoimmune diseases."