Some say that early vitamins lead to allergies. We're not too sure about that. It could very well be cause and effect or just a link. More research on this still needs to be done.
Then there's the hygiene hypothesis theory. It goes that there has been an increase in allergies because we're just too clean.
There has been a study, where 2800 school children in Ecuador showed that allergies were less with people injected with worms. The point of this was that the body would be busy dealing with the worms.
Other studies include children who attend daycare tend to be less likely to develop leukemia or Hodgkin's lymphoma.
Dairy farmers have been shown to be about five times less likely to develop lung cancer. The theory here is that they breathe manure dust containing bacteria.
Image source: Wikipedia.com
Finally, one more study has seemed to show that cotton factory workers have lower rates of some cancers, including lung. The reason came up here was that the endotoxins tend to rev up the immune system.