To be fair, miasma theory made sense and was completely logical, before microscopes. They knew about actual lethal gases, and ones that had different effects on the body. They knew there were gases they couldn't yet detect, as they were still discovering new ones. They knew rotting stuff produced all kind of gases and stenchs. Hell, the theory is still pretty much valid for radon, though for reasons they couldn't have possibly imagined. And at the same time, the idea that animals could get small enough to be invisible, and to invade the human body, was absurd. It came as quite a surprise when the microscope was invented and every single drop of water turned out to be a whole ecosystem teeming with life, and we turned out to be precariously balanced colonies of microscopic cells, also inhabited by a whole bacterial ecosystem teeming with both friends and foes. And then came viruses, which are downright absurd to the point that we're still figuring out where they fit in the tree of life, and whether they're alive or not. And prions, which are the stuff of horror science fiction yet completely logical when you think about how proteins work, and how easily they might sometimes not. Miasma theory wasn't correct, sure, but it was definitely simpler, and made much more sense than the mess reality turned out to be.