There was a good show on NOVA or Frontline or something about all the Soviet research and development that went on after they and the US signed non germ-war treaties. The Soviet expert, now in the US, developed something that was easily transmissable and gave minor cold symptoms when you got it, but what you got was decay of brain myelin - yup, instant, transmissable Alzheimer's, uncurable.

Tom Clancy has already devoted no less than two of his great big books to this kind of attack, in one of 'em evil Muslims prepare to attack the US with weaponized Ebola. In the other, evil environmentalists prepare to destroy 99.99% of the world's population with a derivative of Ebola. Makes it look pretty do-able. In the case of the evil Muslims they depend on the will of Allah. In the case of the environmentalists they have already developed an antidote just for themselves.

The problem is not that it's impossible, the problem is that it is possible, and not that expensive either

The reason it doesn't happen is severalfold: Too hard to control, too easy to be retaliated upon, not that much of an accepted terrorist tactic yet, though it has been tried in Japan and upstate Oregon.

Lastly, within reach is genetic biowarfare, synthesis of toxins that kill or cripple only certain genetic combinations. In "Earth, Final Conflict" such a poison was imagineered to take out just one person.

Happy Easter!