Terrain theory isn't completely wrong, in that overall health does indeed play a huge part in disease, and also that the vast majority of bacteria and viruses are harmless or even necessary. In that sense, terrain theory and germ theory aren't opposed, but work together: Pathogens are more likely to become established in a body that is already weakened. On the other hand, there is absolutely no scientific basis for terrain theory's primary thesis, that "microzymes" lie dormant in human cells, waiting to turn into pathogens when conditions are unhealthy. I suspect many people who subscribe to "Terrain theory" don't really understand what it is. It doesn't simply mean "take care of your body and you won't get sick."
Here's a decent overview.
The post above with the two fish bowls is a bit of a
straw-man and has nothing to do with terrain theory, as it fits quite nicely with germ theory as well. Anybody who accepts the science on pathogens would agree that the fish doesn't need vaccinated; it needs a better habitat.
Germ theory is extremely well documented, as the effectiveness of antibiotics and vaccinations demonstrates. Know anybody who has died of strep throat or smallpox lately?