> it's distinct from hygiene.

Mold

Not to mention that any bacteria thrives in more humid environments. They aren't so good at keeping moist. This is true for a lot of things, especially the smaller the thing is, including bugs. Higher humidity definitely makes good hygiene more difficult.

Why do you think bathrooms have fans? That'd be a lot of effort to deal with farts.

Open the window or run the aircon.

This is a hotel room, you would need the last hundred guests to have done that, not yourself

Considering this and your other comments I really think you need to think a bit deeper about your answers. I believe in you, just ask "and then what happens" and I'm positive you'll figure it out.

I've both opened windows and ran aircons. (Though I try to avoid doing both at the same time.)

Nothing bad happened.

  > Nothing bad happened.

Keep at it, you're almost there. You just forgot about one important variable: time

If done both for a long time over many years.

If the increased humidity promotes mold growth, then yes, it's a hygiene issue.

Yes. Though trapping humidity in the bathroom doesn't make it go away, and you have to open the door to get in and out of the bathroom, and that lets the humidity escape.

The hotel will typically have an extractor running in the bathroom, wired to the light switch.