[Inziladun]

I had another thought on this.

If the workings of "evil" diseases and poisons were directed toward the spirit rather than the body, that would explain why neither Morgoth nor Sauron had any apparent fear that using the plague-weapons could have a negative impact on their own troops.
Then again, what about the Men in the service of evil? While the Orcs seem as a race to be evil-inclined, that doesn't seem to be the case with Men under Morgoth and Sauron. Did Sauron simply not care if his Men died? Maybe he found some way to blame it on the West.