I remember a study done in the UK with Crohns disease where patients were "infected" with an internal parasite which does not normally infect humans and will die off in a couple of weeks. The study seem to show a decrease in the symptoms of Crohns. The theory is that our immune systems evolved to protect against parasites but with modern clean living the immune system has begun attacking what it is suppose to protect. It's kind of like a large standing army with no external enemies seeing enemies in those it is meant to protect.
I can't find that study but there is a company in Germany I believe called Ovamed that using this therapy. If you go to the link there is a Tab at the top labeled TSO which goes into more detail.
http://www.ovamed.de/tso/theory
Theory, Basic Science and Inventors
snip: Over the last decade several research teams have uncovered a possible explanation for the dramatic rise in autoimmune disease and allergy that has occurred in industrialized countries. Many diseases such as Crohns disease, ulcerative colitis, multiple sclerosis, rheumatoid arthritis, insulin-dependent (Type 1) diabetes and asthma became much more prevalent during the last half of the twentieth century (1). The increase in prevalence for these diseases occurred in conjunction with increases in standard of living and hygienic conditions. These types of immune-mediated diseases remain rare in less developed tropical countries. It is clear that a change in our environment has promoted development of immune-mediated disease. Many changes in our environment have occurred. One of the most significant, from an immunologic standpoint, is the eradication of exposure to helminthic parasites (2). These complex organisms have far-reaching effects on the immune system that could prevent development or inhibit expression of autoimmune disease (3).Basic studies using animal models of human disease have shown that exposure to helminths can prevent colitis (1, 4-7), autoimmune diabetes (8,9), and autoimmune encephalitis (10, 11). Recently, investigators at the University of Iowa have shown that exposure to intestinal worms can reverse established colitis in IL-10 deficient mice (12). These studies suggest that exposure to helminths may be able to prevent or treat human disease.