Thieves oil on scarves protected people from plague.

They traded wealth to French Alchemists for the knowledge of antivirals.

To go back further - in the Mahabaratas, i believe (i get the names of those era of books confused) - they talk about the gods fighting with biological weapons.

So if that were a thing, then there would be an opposite flow in the direction of cures. Otherwise - there would only be one fight. And that wasn't the case.