Copilot failed miserably on this one, maybe still worth considering.

Terrain theory is wrong, like most conspiracies it starts with half a truth that is only interesting if you don't understand the real truth and then veers into stupidity quickly.

Of course the feedback for your inaccurate world view in this case isn't quite as tight as say, believing you can fly. Unless you become a disease research scientist could easily go to your grave without life ever forcing you to see the truth. That is true even if your false belief is what ends up killing you.

For anyone just exposed to this and wondering.

Germ theory says germs make you sick. Germ theory understands that your other health context changes your immune system effectiveness. Germ theory understands and has studied the fact that existing stress affects how likely you are to experience symptoms when exposed to germs. The reason is that stress on your body affects immune system effectiveness which when low makes it easier for germs to replicate in your body making you sick.

Terrain theory says the environment in your body causes the germs to magically transmute into a pathogenic form if your body is stressed.

There is basic "look at it in a microscope" evidence for our immune systems weakening during stress. No terrain theory supporter can show what changes when the germs transform into pathogens based on the environment in our body.