One of the more strange sounding books that comes up when I search my library's catalog for books on autoimmune diseases is An Epidemic of Absence, which I have not read. It takes the cleanliness hypothesis- the idea that the immune system makes more errors and creates more disease when there are fewer antigens in the environment (an antigen is any surface that antibodies may target)- and blows it further.
According to the book jacket and reviews, it claims that the absence of internal parasites is bad for us!
So I didn't read it.
However, I was reminded of it when I came across this study today. The article about the study mentions that type 1 diabetes rates are lower in areas where it is common for humans to have worms. For the study, the researchers took worms from sheep bile ducts, and then injected the worms into a strain of mice that's been bred to have autoimmune diabetes. In the control group, 80% of the mice developed diabetes; in the group that was given worms, 16% developed diabetes.