germ theory
microorganisms and pathogens cause diseases. its been demonstrated countless times through microscopy, Koch’s postulates, vaccines, etc etc. example: HIV causes aids, mycobacterium tuberculosis causes TB, SARS-CoV-2 causes covid 19.
the argument that disease come from imbalances in the body, and that germs are more like opportunists rather than primary causes make less sense to me. however, two people might be exposed to the same virus, but one gets very sick and the other doesnt. that does depend on your "terrain" but it doesnt get to the root of the problem. so yes, the host determines stuff like susceptibility and severity, but its pathogens that actually cause the disease.