Bio-warfare In Middle-earth

[Leaf]

Originally Posted by Morthoron

Plagues were once considered magic or "God's Wrath".[...]

Maybe that's what "happend" in this case as well. I think there's the possibility that those plagues might have been natural phenomena that were wrongly associated with Morgoth/Sauron by the people of the West. All we get is their (reasonably biased) point of view which turned into lore and, at some point, was written down by gondorian scribes as history.¹ It's entirely reasonable to suspect that, due to the existing threat and dire circumstances, all sorts of disastrous events were mystified and (re)interpreted as of evil and unnatural origin.

There's, of course, no way of proving that this might be the case, but I think that this thought is interesting nonetheless.

Edit:

1: You can find a way more detailed description of this process in the Note on the Shire records in the prologue of the Lord of the Rings. This text explains how the appendices became part of the recorded history.