A biological weapon is a living organism, or a substance (e.g., a toxin) produced by a living organism, which can be disseminated in such a manner as to cause sickness (i.e., casualty-producing agent) and death (i.e., lethal agent) in a large population. While "population" is usually focused on human beings, biological weapons can be targeted on agricultural animals or crops. An overlap agent affects both people and agriculture.
They are considered weapons of mass destruction. The use of biological weapons was prohibited by the 1925 Geneva Protocol. The Convention on the Prohibition of the Development, Production and Stockpiling of Bacteriological (Biological) and Toxin Weapons and on Their Destruction — more commonly known as the Biological and Toxin Weapons Convention (BTWC) or Biological Weapons Convention — went into force in March 1975.
Inherent dual use problem
One of the challenges of biological weapons control is that organisms that can be used as a disease-producing weapon also can produce naturally occurring disease, so there are quite legitimate needs in medicine and biological sciences to work with the identical organisms. Disease-producing organisms may be prepared in moderately large quantities for developing vaccines and drug testing; BW technology is usually dual-use. The determination that a given effort is for warfare, rather than research or treatment, requires evaluating the quantities involved, and the presence of means for delivering the materials as weapons.
As opposed to the Chemical Weapons Convention, the BWTC does not explicitly list agents of concern. Useful references for organisms deemed as biological threats is the U.S. Select Agent Program, which regulates laboratories handling such weapons, and the CDC Bioterrorism Agents-Disease list, which uses some broader hazard criteria.
Dual-use, as well, is a good deal of equipment for mass culture of biological organisms. For certain agents, other than for safeguards for the staff, there would be little difference between a mass culture system for biological warfare agents and a microbrewery for beer. Industrial equipment that merits greater scrutiny, although still dual-use, include large-volume centrifuges (usually refrigerated), lyophilizers for freeze-drying, and equipment for making fine powders and neutralizing the electrostatic charge on those powders. Even some equipment that could be used to deliver the agent in a war or terrorism situation are dual-use, such as insecticide sprayers, including crop-dusting aircraft. Only bombs and warheads designed to disperse biological agents without destroying them are unquestionably weapons.
While crude attempts at biological warfare using corpses and other infected material go into antiquity, serious experimentation by industrial states began in 1932 in Japan's Unit 731, in the U.K. and U.S. circa 1942, and certainly in the Soviet Union, although the dates are less clear. There is relatively little evidence that Nazi Germany worked on the mass distribution of pathogenic organisms, although they did experiment in infections with unwilling individuals.
The 1972 Biological Weapons and Toxins Convention shut down official research in offensive biological warfare, although it permitted defensive research on detecting biological attacks and providing defenses such as filters and decontamination techniques.
The Soviet Union may have continued offensive work in secret, under the cover of an organization called Biopreparat. Details were provided by Ken Alibek, the deputy director, when he defected to the West. 
Characteristics of organisms for biological warfare
As long as biological agents were being considered as weapons of nation-states, some disease-producing organism were inappropriate for military use. Perhaps the most important factor in eliminating an organism from consideration is a significant ability to spread from person to person. Direct human spread could be uncontrollable, and easily could endanger the country that originated the use.
That restriction, however, is not applicable to some terrorist organizations, and the ability to create self-sustaining epidenics may even be desirable.
- understood effect (casualty-producing vs. lethal)
Immunizations are available for some potential threats, such as anthrax; military personnel have been vaccinated.
- Protocol for the Prohibition of the Use in War of Asphyxiating, Poisonous or other Gases, and of Bacteriological Methods of Warfare, Geneva, 17 June 1925
- Biological and Toxin Weapons Convention (BTWC) website
- "Biopreparat", Federation of American Scientists
- Ken Alibek, Biohazard