Chemical and biological agents can spread through the air, water and food supplies.  See more biological and chemical warfare pictures.

During the gulf war, the threat of Iraqi chemical and biological weapons felt very real, because it was known that Iraq had done extensive research on these weapons. In the wake of the September 11 terrorist attacks, the threat feels very real again. A chemical or biological weapon used in a large city would kill thousands of people.

In this edition of HowStuffWorks, you will learn how chemical and biological weapons really work, how they might be deployed and what the actual threats are.