Potentially, biological weapons can cause many thousands of casualties with the use of a very small quantity of material. In terms of a threat to human life they are on a scale comparable to a nuclear weapon as a weapon of mass destruction, and some countries view them as the strategic equivalent.

Biological weapons (BW) can be developed from living organisms (for example, bacteria and viruses) or toxins (poisons) obtained from these organisms. Given the right technological expertise biological weapons are cheaper and easier to produce than nuclear weapons. Recent developments in biotechnology in civil industry, mainly in the pharmaceutical and veterinary medicine sectors, have made possible the easier production, storage, and weaponisation of some types of pathogenic organisms. Until the mid-1970s, while BW had some military value, the state of scientific and technological development was such that their effects were too unpredictable and the problems of their storage and handling too great; consequently, other weapons conferred greater advantages. The advances in civil industry, mainly in the past twenty years, have made possible the production of weapons that are more effective and have more predictable results. In particular, biotechnological capabilities have now advanced to the point that bacteria, viruses and toxins can be produced by synthetic means.

It is also easier to hide a BW program in civilian research and production facilities than either a nuclear or chemical one. The two biggest known post-World War II clandestine programs, in Russia (beginning in the days of the Soviet Union) and Iraq, used this technique. The hardest part of a BW program to conceal is the weapons end of the process, when the organism or toxin is put into a missile warhead, bomb, artillery round, or aerial spray tank. However, this activity can be done shortly prior to the intended use to achieve maximum surprise. Most at risk to these deadly weapons are unprotected civilian populations. For example, studies have shown that a missile delivering 30 kilograms of anthrax spores overhead an urban area could kill between 80,000 and 100,000 people if they had no special protection in an area of some 10 square kilometres. In comparison, a 12.5 kiloton (Hiroshima-size) nuclear weapon delivered over a similar area could kill between 23,000 and 80,000 people (but also cause severe structural damage). For a chemical agent to be delivered to achieve a similar casualty toll to the BW delivery described above, many more kilograms of agent would be required. For example, even three hundred kilograms of a highly lethal chemical agent, such as sarin nerve gas, delivered over a similar target could kill perhaps only 80 to 200 people and cover an area only a fraction of that covered by ten times less (in weight) of anthrax.

The Biological Weapons Convention (BWC), which entered into force in 1975, bans the research, development, production, stockpiling, or acquisition of biological and toxin weapons. The convention also bans delivery systems specifically designed for such weapons. While the BWC deals only with possession of these weapons, use in warfare was banned by the 1925 Geneva Protocol (which also similarly bans the use of chemical weapons); although some States parties entered reservations asserting the right to use such weapons if an enemy State used them first, an absolute ban now exists as a matter of customary law. The 1925 Protocol was itself derived from an ancient customary law of war restricting the use of “poisonous” weapons or substances in armed conflict that had first been codified in the Hague Conventions of 1899 and 1907. To date 155 countries have ratified or acceded to the BWC.

The BWC contains no provision for monitoring or inspection, but there is a nonbinding “confidence-building” regime under which States may make declarations about their facilities that handle highly pathogenic organisms and give lists of publications which deal with them. Between 1993 and 2001 there was an attempt to negotiate a legally binding protocol for verification of compliance, including inspections, but consensus on a text was not achieved. The tension between the requirement for intrusive inspection measures and political, economic and other national security demands prevented a successful outcome.

While the United States and Britain had dismantled their BW programs well before the BWC came into force, it came to light by 1990 that the Soviet Union had been running an extensive illegal BW program. They took advantage of the more recent developments in biotechnology to carry their program forward to a much more advanced level. In April 1992, President Boris Yeltsin of Russia publicly admitted to the illegal BW program and announced a decree ending it. Convincing proof that the program has been fully dismantled has yet to be provided.

Iraq, although a signatory to the BWC, never ratified it and developed a comprehensive program using viruses, bacteria, and toxins. By the time of Operation Desert Storm in 1991, which resulted in the liberation of Kuwait, Iraq had missiles, aerial bombs, and artillery filled with biological agents and ready to use. They also had research and development programs on new agents and delivery means in progress. Neither the United States nor Britain knew the scale and scope of the program, but both took precautions, such as anthrax vaccination programs for the troops and prophylactics against nerve agent attacks. Britain also fielded a biological agent warning and detection system. Coalition bombing targeted chemical weapons plants but left the main BW sites untouched as the coalition was unaware of them.

Iraq only admitted to having a BW program in 1995 after more than four years of investigations by United Nations inspectors. However, Iraq still continued to hide important aspects of this weapons program for several years. After the coalition invasion of Iraq in March 2003, no evidence of a continuing program was discovered

Other smaller programs existed or still exist in other parts of the world. An example of one, now ended, was that in South Africa run by the apartheid regime until the early 1990s. This was a relatively small program aimed for assassination missions. However, it used advanced biotechnological techniques and it is possible that some of those who worked on the program have exported their skills elsewhere.

Biological weapons entered the realm of terrorism with the 2001 anthrax attack in the U.S. delivered through the mail system that affected people in several states. Five people died and 22 people were seriously ill. The perpetrator of this attack remains unknown. There is clear evidence that some terrorist groups, such as al-Qaeda and the Japanese Aum Shinrikyo, considered and experimented with biological weapons but there have been no other lethal terrorist attacks.