I keep hearing this supposedly revelatory anecdote, as if caused by stress and bacterial infections are mutually exclusive factors.

It's well known that chronic stress weakens the immune system, and makes it easier for a bacterial infection to take hold.

So a more complete hypothesis is that stress can be the cause, whereas weakened immunity and consequent bacterial infection is the mechanism.

Of course, anything else that causes the immune system to be weakened can also be a cause.

Stress was neither necessary nor sufficient to produce the ulcers; it may have been involved in some cases, but it was not the "cause."

But H. pylori was also not sufficient to produce the ulcers; most people in the world have it, but far fewer develop ulcers.

As user interpenetrate has articulated in more depth, the error is in the need to identify a single, simple "cause".

I don't think that's much more useful than saying a flu is caused by stress, is it?

It's not useful to try and explain an illness by a singular cause, when multiple factors are extant and important.

Susceptibility to flu is influenced by immune system fortitude; that's uncontroversial. That stress is one factor (among several) that can weaken the immune system in an otherwise healthy person, is surely useful to consider when making life choices.

Most people with this bacteria do not develop ulcers. Psychological stress does seem to increase development of ulcers -- it's a contributing factor. https://www.ncbi.nlm.nih.gov/pubmed/25111233

I notice in myself that I often get a sore throat after being stressed and not sleeping enough.