Yes that's another classic. Just be careful about whether you make it possible to de-infest people/beings. If you can, and you don't, suddenly a gigantic blazing moral question appears in the heavens and won't go away, like some baleful comet. Even if you make it so say, killing the source de-infests the remaining people, then as soon as you learn that it becomes a moral imperative to kill as few infested as possible, which can put a real damper on things. I think the best way of doing it is to say "If you're fully infected, it's over, you're done, no going back, but we can stop future infections by doing X or Y".