From the mother of all knowledge, one of my biggest obessions, and the cause of most of my procrastination:

Moderate hookworm infections have been demonstrated to have beneficial effects on their hosts. Research at the University of Nottingham conducted in Ethiopia has demonstrated that people with hookworm infections are half as likely to experience asthma^[1] or hay fever.^[2]

The theory is that our immune systems evolved under constant assault from a variety of parasites, most of which have to modulate our immune response to succeed. That is, they have to down regulate the response that would otherwise attack them. Evolving with a down-regulated immune system means that in the absence of those down-regulating parasites our immune systems often attack our own tissues, leading to asthma, hay fever, IBD, colitis, Crohn’s and perhaps other autoimmune diseases. Hence the increase in autoimmune diseases in the relatively clean and sterile industrialized world.