I think when people are too anal about bacteria their stomachs can end up weak because they never have to deal with any bacteria.
That's my theory anyway backed up by solid anecdotal evidence that I have a friend who is over-the-top about bacteria and she gets gastroenteritis every single year. And it's getting worse each year - been to hospital the last two times. And she's not careful because of the gastro, the gastro came later.
Whereas my stomach is super tuff.