Contrary to popular opinion, people (and animals) are supposed to have parasites. In fact, many scientists believe that there have been negative consequences to ridding the human gut of parasites.
Various studies suggest that parasites can regulate the immune system in ways that prevent it from going "wild" and attacking healthy tissue.
Joel Weinstock at Tufts Medical Center in Boston has been using parasitic worms to treat people with ulcerative colitis. He says drug companies are now trying to create parasites that would actually be approved by the FDA for treating inflammatory bowel disease.
While common in the U.S., inflammatory bowel disease is rare in places where parasites are common.
Read NPR article