What it is: Highly infectious disease caused by touching or breathing in Bacillus anthracis spores.
How it works: The cutaneous form causes necrotic lesions. Inhalation anthrax initially resembles a cold, but progresses to cause organ failure.
Why it's a threat: Cutaneous anthrax kills 20 percent of those infected. Inhalation anthrax is usually fatal, even with treatment.
What it is: Toxin produced by the Clostridium botulinum bacteria.
What it does: Paralyzes muscles, eventually leading to death from respiratory failure.
Why it's a threat: Although treatable in its natural form, weaponized botulism toxin is far more deadly.
What it is: Historical scourge caused by Yersinia pestis bacteria, found in rodents and their fleas.
What it does: Causes fever, chest pains, bloody sputum, etc. Absent treatment, it usually results in death.
Why it's a threat: Symptoms may take up to three days to appear - making spread of the disease difficult to control.
What it is: A gas, 2-chloroethane, used during World War I.
What it does: Causes blistering to the eyes, skin and respiratory tracts. Has also been shown to damage DNA.
Why it's a threat: Mustard gas is easy and cheap to make in huge quantities. There is no antidote.
What it is: Toxic, gaseous agent used in the plastic and pesticide industries.
What it does: A choking agent, phosgene eventually causes the lungs to fill with fluid.
Why it's a threat: Phosgene, like mustard gas, is easy to produce. And it has an even deadlier track record: In World War I, phosgene killed more people than any other chemical agent.
What it is: Odorless, colorless, tasteless gas that attacks the nervous system.
What it does: Sarin disrupts the "off switch" for certain glands and muscles, causing them to fire constantly. It exhausts the body until breathing can't be sustained.
Why it's a threat: Sarin is a proven killer. In 1995, 12 died when the Aum Shinrikyo cult attacked a Tokyo subway.