Bacteriology was already developing at that time, first vaccines were already in use (for none other than a viral disease!), and the existence of a pathogen smaller than bacteria was already hypothesized. The response to Spanish flu was also eerily similar to what was recommended in the first year of COVID: https://en.m.wikipedia.org/wiki/Spanish_flu#/media/File%3AIl....

All that aside, your comment does not really prove that "nature cannot come up with something so transmissive", which was the claim here.