Smallpox, which the only remaining samples exists in a couple of secure facilities controled by superpowers for use making vaccinations in case they are wrong about their only being a few samples controled by superpowers. Everyone with an ounce of sense knows bioweapons infect both sides and nuetral parties who are no longer neutral once you infect them. It like mustard gas but worse no one other than suicidal terror groups want them and they dont have the facilities equipment samples or knowhow.