and it is guaranteed that some diseases were blamed on bacteria or virus, but were caused by poisons either industrial or pharmaceutical. Poisons weakening the immune system would also certainly cause outbreaks.

The medical system straight up lying forever is why people created the terrain theory imho. Questioning everything when the medical system is so consistently lying to everyone isn't a bad thing. I found out that chicken pox and small pox (And shingles) are all likely the same disease by someone who believed terrain theory. The main whole in terrain theory is natural immunity. Plenty of diseases are once in a lifetime, maybe twice if the immune system gets overwhelmed by something, and diseases from poisons and toxins would keep reoccurring based on exposure.