Two IBS studies have noted “a history of abdominal pain or bowel troubles in first-degree relatives appears to be independently associated with both persistent and fluctuating irritable bowel syndrome", and, “having a mother with IBS and having a father with IBS are independent predictors of irritable bowel status”. With Crohn’s it’s also been noted by one study that, “eight patients had a family history (3 first degree and 5 second degree relatives)” of CD.

Considering Helicobacter pylori, for example, it’s been suggested that, “infected parents, in particular mothers may play a key role in transmission of H. pylori to the child”. And with Streptococcus mutans it's been noted: "The strains of the same serotypes of S. mutans which possessed similar mutacin patterns were predominantly detected in the siblings and mothers of each subject."

So, if MAP is a major cause of both irritable bowel syndrome and Crohn’s disease, there’s a strong possibility of a parental or first/second-degree-relative association between CD and IBS sufferers in the same family. A population-based study may reveal such a link.