Mycobacterium avium subspecies paratuberculosis (MAP) has been a contender as a cause of Crohn's for a while, but nothing conclusive has been determined. It causes the Crohn's-like Johne’s disease in cattle, and people with Crohn's are significantly more likely to have MAP in their intestines than non-sufferers. The theory is that susceptible people who drink MAP-contaminated milk will may develop Crohn's. Pasteurisation of milk does not eliminate MAP, but ultra-heat treatment, as used in long-life milks, does.The Western Farm Press has an interview with veterinary professor William Davis which outlines why he believes the MAP link to Crohn's should be investigated further.
"We know the incidence of Johne’s is on the rise, as is the incidence of Crohn’s,” said Davis, whose research has included collaboration with physicians and immunologists. "The problem is too important and the theory is sufficiently plausible that we need to be taking a closer look. More research is necessary to resolve exactly what the link is.”He also notes that preliminary work has found that his vaccine against MAP in cattle has seemed effective. If it is successful then this may eliminate MAP entering the human food chain, and possibly reducing the incidence of Crohn's.
Note, however, that antibiotics to combat MAP have thus far been ineffective against Crohn's, and the MAP link is far from proven.