Chemical and Biological Weapons
The worldwide stockpile of biological and chemical weapons is about 61,000 metric tons — enough to kill 60 billion people. There are only 6.5 billion people.
All too often, discussions about WMDs focus just on nuclear weapons. However, chemical and biological weapons can be just as deadly, yet more insidious. According to the Journal of the American Medical Association, "Botulinum toxin is the most poisonous substance known. A single gram [1/30th of an ounce] evenly dispersed and inhaled, could kill more than 1 million people..." That would make the human lethal dose 1/30,000,000th of an ounce.
Biological weapons employ viruses, bacteria and other germs to produce diseases, which disable and kill people. The Class A agents described below are of critical concern from a bioterrorism perspective.
The British used smallpox as a weapon in the mid-Eighteenth Century French and Indian War. Under the pretext of friendship, the British gave blankets from their own smallpox victims to Indians who were sympathetic to the French. The Indians lacked immunity from smallpox and they suffered a devastating outbreak, enabling the English to defeat the French at Fort Ticonderoga. The discontinuation of the smallpox vaccination after the disease was eradicated in 1980 has made the world vulnerable again to its use as a weapon.
Smallpox victim. Source: CDC, Centers for Disease Control and Prevention
Kurdish boy gassed in Halabja, Iraq on March 16, 1988. 5,000 people were killed; 75% were women and children. Their skin was discolored, their eyes open and staring where they had not recessed into their sockets, a grayish mucous was oozing from their mouths and their fingers were clenched. Photo courtesy of Kurdish Democratic Party.
Sarin is a chemical warfare agent, which targets the nervous system and inhibits it from functioning properly. Nerve agents are the most toxic and rapidly acting of the known chemical warfare agents. Sarin is 26 times as deadly as cyanide gas, and a drop the size of a pinhead is sufficient to kill. Choking and bleeding from the nose and mouth commence within a few seconds after exposure to it and can lead to death. It was used by Saddam Hussein along with other chemical warfare agents (Mustard Gas, Cyanide and VX nerve gas) during the Iran-Iraq War in the 1980s, and against the Kurds in 1988. Sarin was also used by the terrorist group Aum Shinrikyo (Supreme Truth Cult) in the Tokyo subway attack on March 20, 1995.
Cutaneous Anthrax. Source: CDC, Centers for Disease Control and Prevention
“Less than one millionth of a gram [1/30,000,000 of an ounce of inhaled anthrax] is invariably fatal within five days to a week after exposure.” 
Anthrax spores occur naturally around the world in soil and in certain animals, and they can be produced for biological warfare. When dispersed by exploded devices, the spores will survive the explosion and retain their ability to infect people. Anthrax can remain viable in soil for decades.
“Botulinum Toxin as a Biological Weapon,” JAMA (Journal of the American Medical Association), Vol. 285, No. 8, February 28, 2001, http://www.fas.org/biosecurity/resource/agents.htm. March 27, 2005.
 FAS (Federation of American Scientists), "Biological Warfare Agents (Partial List)," www.fas.org/nuke/intro/bw/agent.htm, (28 March 2005).