@Nematoda Not necessarily. Long before germ theory, or even an understanding of how disease spreads, we as a species have been using animals (living or dead) to maliciously spread disease as a form of biological warfare.
Get the rancid corpse of an animal, often a horse or cow, & lob it into a city's waterway upstream, or over the walls via catapult, trebuchet, or something similar.
People realised that you got sick from being around the rotting corpses, so it stood to reason that corpses had something on them that caused sickness. Be that evil spirits, bad vibes, or as we know today bacteria & viruses.
On the other hand, I would like for there to be another Otherworlder in this story. They might be locked up & being tortured for information by the BBEG/BBEE & need saving, or maybe they're just a malicious prick & are using their knowledge to gain status & power.
Hell maybe they're not even from MC's world or time. Who's to say that they lived on the same Earth? If you've got reincarnation I don't see why you can't pull from multiple universes.