MAP is a mycobacterium known to cause serious illnesses in mammals – just like its cousin, M. tuberculosis, which causes pulmonary tuberculosis.

"Just because an infectious agent is present doesn’t mean everyone will get sick from it,” Davis said, pointing to M. tuberculosis. "Ninety percent of the world’s population infected with M. tuberculosis exhibit no symptoms because their immune systems control infection.

"But we also know that if their immune systems get weakened, such as in the elderly or people with HIV, the disease can become active,” he said. "The situation is most likely the same for infection with MAP.”

‘A huge deal’

Skeptics insist that MAP be consistently found in all Crohn's patients before they will consider it as a potential cause of the disease, according to medical literature critical of the MAP-link theory. But physician and researcher William Chamberlin - who sees Crohn’s patients almost daily - doesn’t need that kind of evidence, he said.

"This is a classic case of the 800 pound gorilla in the living room that’s easier to ignore than to do something about,” said Chamberlin, a gastroenterologist who, in 2011, co-authored a review in the journal Clinical Immunology concluding that Crohn’s is caused by infections, not an autoimmune disorder.

Backing the 2008 report by the American Academy of Microbiology suggesting that Crohn’s isn’t a single disease but a syndrome with different causes, "MAP is but one of them,” said Chamberlin in an interview.

"The idea that MAP triggers a syndrome in humans that we call Crohn’s is a huge deal,” he said. "From a medical treatment perspective, we need to be strengthening the body’s defense, not suppressing it with steroids and other drugs.”