There was a report a few years back suggesting that schizophrenia was rarer some centuries ago, and that possibly the disease had an infectious component.
The suggested vector for the disease is man's best friend, the noble cat. We have some indications that toxoplasma can make changes in the human brain as well as that of mice, so perhaps we have extra reasons to worry. This isn't certain. But wash carefully, OK?
Suppose we knew it to be true that toxo caused schizophrenia, and in some people turned them into mass killers as a result. Would we turn medieval on cats?
Muddy the waters a bit. Suppose toxo was sort-of symbiotic (like gut bacteria that help us digest food) and had mood regulator effects that tended to suppress human violent impulses in most cases, and only drove people kill-crazy in a few. The net effect would be positive--fewer murders--but the dramatic incidents would outweigh the benefits, because the mechanism is so creepy. It wouldn't be like the horrific traffic accidents that we accept as a side-effect of driving; it would be much more horrible. Even the beneficent effects would seem creepy.
And that creepy horror would extend from the mostly-beneficent toxo to the cat.
What do you think people would do? What would you do?