[rtb]

No-one is claiming that stress synthesizes bacteria, that is a laughable strawman.

We are surrounded by bacteria at all times. If stress were to weaken your immune system, then you might be more easily infected by them. This seems completely plausible to me (likely, even). It is not a great leap to describe this link as stress "causing" the infection, even if it might be better to describe it more precisely. Perhaps "stress meant he couldn't fight off the infection as he normally would have"?