The idea of people using biological weapons is a tricky one, because the populations related to those using the weapons would be just as vulnerable to being wiped out by them as those they were trying to wipe out. That makes them a lot less likely to be used. The smallpox bio-weapons were effective because Europeans tended to already have some degree of immunity to smallpox, and the indigenous Americans did not, and the indigenous populations were fairly isolated from the other populations. This is not the case with modern bio-weapons, although we did see that someone (someone employed by the US government) was able to use anthrax to kill a small number of people through the mail in 2001.