A story I heard from someone that works with the CDC who heard it at a bioterrorism workshop.
The Aum Shinrikyo, a wacky Buddhist-cult, was being hassled by a Japanese judge. They decide to kill him. Since it's hard to buy guns in Japan, they decide to kill him with Anthrax.
1. They obtain some Anthrax, which seems to be surprisingly simple for people to get.
2. They buy a high-rise apartment building, upwind of where the judge lives. This is in Tokyo.
3. One fine, sunny day, they release the Anthrax from the high-rise apartment, upwind of the judge's apartment. The Aum Shinrikyo, not being versed in germ warfare, didn't realize that releasing stuff over a city during the day doesn't work, as the heat from the concrete and asphalt heated by the sun gives rise to convection currents. Anthrax spores are lifted up and out of harm's way by the friendly powers of convection.
4. Through the miracle of higher learning and the Internet, the Aum Shinrikyo realize their mistake. They get more Anthrax, and this time, release the spores at night.
5. The Anthrax spores, carried by the breeze, go all over the judge's apartment, as well as through a good chunk of Tokyo. Luckily, the easily-obtained Anthrax was a form used as a vaccine in livestock. A chunk of Tokyo, including the judge, may have been mass-vaccinated against Anthrax, but no one gets sick.
6. At this time, the Aum Shinrikyo decide it's much more fun to use Sarin gas in subways, and succeed in killing innocent people.
It was the guess of the speaker at the conference that if they got the right kind of Anthrax, at least 10,000 people would have died.