It may not relate to you, but I know of a lot of people where this is a major problem. I've also know of cases where stress and constipation work together; there are cases where is it stress related, such as, "a pain in the (censored)".... when another person maybe an irritant to someone.
As far the mirobe world, it is rather complex! We're taught that a compromised immune system is the problem; unfortunately, this isn't always the case and there are people who have intact immune systems that have become infected. I know that I've posted a few cases.... fungi is something that comes to mind as an example.