It has been estimated that a small plane carrying 10kg of anthrax spores - which could theoretically be disguised as a crate of duty free - would be enough to kill three million people if sprayed over a city. Its effect would depend, however, on weather and windspeed, the general health of the target population and the availability of antibiotics to fight the infection. It could be placed in drinks or food but this would kill comparatively small numbers.
ANTHRAX is not a biological weapon of mass destruction killing everyone who has contact with it. That dubious honour belongs to the nerve gas, Sarin. It is, however, a lethal poison. Like other poisons, whether it kills depends on the dose absorbed and the strength of the victim's immunity.