Bio-warfare In Middle-earth

[Ibrîniðilpathânezel]

I think it's also possible that he didn't do it again because he had achieved one of the goals of sending out a plague: he had diminished the numbers of his foes to the point that the various groups were effectively isolated from one another. Such separation tends to keep people thinking more about themselves and less about others, so that when the time comes to band together against a common enemy, they don't want to, either because they've grown suspicious of outsiders, or because they feel that they barely have the numbers to defend themselves, and not enough to send help elsewhere. Had Gandalf not intervened in Rohan, that end would have been achieved.