Anthrax was the first bio-terrorist weapon of choice and, unfortunately, it was successful. It wasn't the horrendous disaster of 9-1-1, but for all intents and purposes it shut down Washington D.C., for several weeks in 2001.
There are more biological agents out there than we'd like to think about. One of the ones that has always had a fierce reputation is the weapon that uses "the Black Death" as its basis.
Small pox is a killer disease, pure and simple. However, due to the first truly international effort to eradicate a disease, smallpox is now not considered to be an international health threat...unless some idiot uses it for a biological weapon.