
Here's a link to a story RadioNZ ran a few days ago.
It appears that exposure to some minor gut parasites can be protective against diseases like Caeliac Disease, Inflammatory Bowel Disease (Crohn's Disease and Ulcertative Colitis), and Irritable Bowel Syndromme (IBS). There's also some evidence for intestinal worms having a role in protecting against some forms of Arthritis