Terrorists could poison more than 500,000 people, and more than half would die, by putting as little as 10 grams of botulinum toxin in a milk truck. That is the conclusion of a scientific risk analysis which was pulled from publication at the last minute by US government officials in May 2005, but has now been published [LINK]. Full text of publication [here]
Is this a realistic threat? Maybe not. See "Got Toxic Milk - A Rejoinder" [here].