In four months, FBI agents and scientists have unraveled many of the mysteries surrounding the strain of anthrax used in last fall's deadly attacks. The "Ames strain" is now known to be highly virulent, resistant to many vaccines and a perennial favorite of military researchers and bioterrorists.

Not only water supplies are susceptible to premeditated contamination; so too is much of the air we breathe. Skyscrapers, tunnels, subways, and their requisite heating and air-conditioning systems provide targets and also avenues of attack for would-be bioterrorists.