From my College Home Ec days: A TEASPOON of botulinum can kill a horse. A Teaspoon of botulinum could kill a city if properly distributed and applied. Clearly in a WMD, most would be simply dispersed into the environment, but it is an incredibly toxic substance.
:) Think I mentioned the idea with the "an amount the size of a tiny grain of salt can kill several people within an hour" quote. Thanks, though.