Specifically, there is a reason the US military still vaccinates for smallpox, a virus that was eradicated world-wide 50 years ago. As a civilian, you can't get it even if you wanted it, but as a soldier, you have to take it even if you don't want it.
There is also the legend of sieging cities by catapulting diseased cows over the city walls. And if anyone knows the citation for that story, I'd love to see it.