Histroically we've tended to think of intestinal parasites (we're talking about worms) as being a bad thing, with only negative health effects.  This is still true, for the most part.  There is an increasing amount of evidence that exposure to some parasites may offer protection from some autoimmune diseases.

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