[pmulcahy11b]

The problem with biological warfare, especially with modern agents (for lack of a better word in my brain right now) is that they can (and this is an actual military term) "boomerang." They can come back to your own troops and lands, sometimes decades later. Those fancy shots you gave your people against your version of the illness may not protect them 6 months later (or whatever time period). And viruses, in particular, mutate fast -- what you let loose on the battlefield two weeks ago might blow right through your special immunizations when it comes back to you. Or become harmless. Or turn into something worse -- "I just ate breakfast with him an hour ago and he's already dead? Does that mean I'm sick too?"

In every OPLAN (Yes, I seen a few complete and incomplete ones in my time in the Army), biowar was the warfare with the fewest realistic options, even more dangerous to the planet than a total nuclear exchange, and something you never want to deal with. Because ultimately. you can't.