What is a biological weapon?

A biological weapon uses disease-producing agents like bacteria , viruses , or toxins against people, animals, or plants. Though they can cause mass deaths, they don't destroy infrastructure like nuclear weapons. Because they can start pandemics and are hard to control, most countries have agreed to ban them. The Biological Weapons Convention (BWC) prohibits the use, development, and stockpiling of biological weapons. As of 2013, 180 states and Taiwan had signed the BWC, with 170 of those states and Taiwan ratifying the treaty. However, biological weapons programs are easily hidden, and the weapons themselves are not as expensive to make as nuclear weapons.

How do biological weapons work?

Biological weapons utilize disease-producing agents like bacteria, viruses, fungi, or toxins against humans, animals, or plants. These agents vary significantly in lethality, incubation period, infectiousness, and treatability. Most lethal biological agents are designed to be delivered as aerosols, causing infection when inhaled. Defenses include protective masks with filters, overgarments, and decontaminants. Effective sensors and alarms can help personnel take protective measures before exposure, and vaccinations can reduce the impact of attacks. Historically, biological weapons have been used to spread disease, causing more deaths than combat in some conflicts. Today, the Biological Weapons Convention (BWC) prohibits member states from developing, testing, producing, stockpiling, or deploying biological weapons.

How are biological weapons different from chemical weapons?

Chemical weapons use toxic chemical agents to cause harm or death to humans, animals, or plants. They come in various forms, such as gases, liquids, or solids, and inflict damage through inhalation, absorption, or ingestion. Biological weapons utilize disease-producing agents like bacteria, viruses, or toxins to harm humans, animals, or plants. While chemical weapons cause harm through direct toxic effects, biological weapons cause harm through disease. Biological weapons are also hard to control and can potentially start widespread pandemics.

What are some historical examples of biological weapons being used?

Biological weapons are disease-producing agents used against humans, animals, or plants. Historically, they have had devastating consequences. In 1346, the Mongols catapulted plague-ridden corpses over the city walls of Kaffa (now Feodosiya, Ukraine), leading to the Black Death in Europe, which claimed 25 million lives. Japan also used biological weapons in China and conducted experiments that killed over 3,000 people. More recently, the AUM Shinrikyo religious sect released anthrax in Tokyo in 1993. In 2001, anthrax-laced letters were sent in the United States, killing five and infecting more than a dozen. These examples highlight the deadly potential and historical use of biological weapons.

What international agreements or rules exist to control biological weapons?

The primary international agreement to control biological weapons is the Biological Weapons Convention (BWC) . Signed in 1972 and effective from 1975, the BWC prohibits the development, production, acquisition, stockpiling, or transfer of biological weapons. As of 2013, 170 states and Taiwan had signed and ratified the BWC. The Australia Group , formed in 1985, is an informal group of nations that works to prevent the export of chemical and biological weapons and the materials used to produce them. By 2013, it had grown to 42 member states. All members are required to sign the Chemical Weapons Convention and the Biological Weapons Convention.

biological weapon

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At a Glance

biological warfare summary

biological weapon

weaponry

Also known as: germ weapon

Written by

Barry R. Schneider

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Britannica Editors

Last updated

Nov. 4, 2026 •History

Top Questions

What is a biological weapon?
A biological weapon uses disease-producing agents like bacteria, viruses, or toxins against people, animals, or plants. Though they can cause mass deaths, they don't destroy infrastructure like nuclear weapons. Because they can start pandemics and are hard to control, most countries have agreed to ban them.

The Biological Weapons Convention (BWC) prohibits the use, development, and stockpiling of biological weapons. As of 2013, 180 states and Taiwan had signed the BWC, with 170 of those states and Taiwan ratifying the treaty. However, biological weapons programs are easily hidden, and the weapons themselves are not as expensive to make as nuclear weapons.
2 Britannica Sources
1. biological weapon
2. 7 Deadliest Weapons in History: Biological weapons
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How do biological weapons work?
Biological weapons utilize disease-producing agents like bacteria, viruses, fungi, or toxins against humans, animals, or plants. These agents vary significantly in lethality, incubation period, infectiousness, and treatability.

Most lethal biological agents are designed to be delivered as aerosols, causing infection when inhaled. Defenses include protective masks with filters, overgarments, and decontaminants. Effective sensors and alarms can help personnel take protective measures before exposure, and vaccinations can reduce the impact of attacks.

Historically, biological weapons have been used to spread disease, causing more deaths than combat in some conflicts. Today, the Biological Weapons Convention (BWC) prohibits member states from developing, testing, producing, stockpiling, or deploying biological weapons.
2 Britannica Sources
1. biological weapon: Biological warfare agents
2. 7 Deadliest Weapons in History: Biological weapons
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How are biological weapons different from chemical weapons?
Chemical weapons use toxic chemical agents to cause harm or death to humans, animals, or plants. They come in various forms, such as gases, liquids, or solids, and inflict damage through inhalation, absorption, or ingestion.

Biological weapons utilize disease-producing agents like bacteria, viruses, or toxins to harm humans, animals, or plants. While chemical weapons cause harm through direct toxic effects, biological weapons cause harm through disease. Biological weapons are also hard to control and can potentially start widespread pandemics.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What are some historical examples of biological weapons being used?
Biological weapons are disease-producing agents used against humans, animals, or plants. Historically, they have had devastating consequences.

In 1346, the Mongols catapulted plague-ridden corpses over the city walls of Kaffa (now Feodosiya, Ukraine), leading to the Black Death in Europe, which claimed 25 million lives. Japan also used biological weapons in China and conducted experiments that killed over 3,000 people.

More recently, the AUM Shinrikyo religious sect released anthrax in Tokyo in 1993. In 2001, anthrax-laced letters were sent in the United States, killing five and infecting more than a dozen. These examples highlight the deadly potential and historical use of biological weapons.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What international agreements or rules exist to control biological weapons?
The primary international agreement to control biological weapons is the Biological Weapons Convention (BWC). Signed in 1972 and effective from 1975, the BWC prohibits the development, production, acquisition, stockpiling, or transfer of biological weapons. As of 2013, 170 states and Taiwan had signed and ratified the BWC.

The Australia Group, formed in 1985, is an informal group of nations that works to prevent the export of chemical and biological weapons and the materials used to produce them. By 2013, it had grown to 42 member states. All members are required to sign the Chemical Weapons Convention and the Biological Weapons Convention.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

biological weapon, any of a number of disease-producing agents—such as bacteria, viruses, rickettsiae, fungi, toxins, or other biological agents—that may be utilized as weapons against humans, animals, or plants.

The direct use of infectious agents and poisons against enemy personnel is an ancient practice in warfare. Indeed, in many conflicts, diseases have been responsible for more deaths than all the employed combat arms combined, even when they have not consciously been used as weapons.

Biological weapons, like chemical weapons, radiological weapons, and nuclear weapons, are commonly referred to as weapons of mass destruction, although the term is not truly appropriate in the case of biological armaments. Lethal biological weapons may be capable of causing mass deaths, but they are incapable of mass destruction of infrastructure, buildings, or equipment. Nevertheless, because of the indiscriminate nature of these weapons—as well as the potential for starting widespread pandemics, the difficulty of controlling disease effects, and the simple fear that they inspire—most countries have agreed to ban the entire class.

As of 2013 a total of 180 states and Taiwan had signed the Biological Weapons Convention (BWC) and 170 of those states and Taiwan had signed and ratified the treaty, which was opened for signature in 1972. Under the terms of the BWC, member states are prohibited from using biological weapons in warfare and from developing, testing, producing, stockpiling, or deploying them. However, a number of states have continued to pursue biological warfare capabilities, seeking a cheaper but still deadly strategic weapon rather than following the more difficult and expensive path to nuclear weapons. In addition, the threat that some deranged individual or terrorist organization will manufacture or steal biological weapons is a growing security concern.

Biological warfare agents

Biological warfare agents differ greatly in the type of organism or toxin used in a weapons system, lethality, length of incubation, infectiousness, stability, and ability to be treated with current vaccines and medicines. There are five different categories of biological agents that could be weaponized and used in warfare or terrorism. These include:

Bacteria—single-cell organisms that cause diseases such as anthrax, brucellosis, tularemia, and plague.
Rickettsiae—microorganisms that resemble bacteria but differ in that they are intracellular parasites that reproduce inside cells. Typhus and Q fever are examples of diseases caused by rickettsia organisms.
Viruses—intracellular parasites, about 1/100 the size of bacteria, that can be weaponized to cause diseases such as Venezuelan equine encephalitis.
Fungi—pathogens that can be weaponized for use against crops to cause such diseases as rice blast, cereal rust, wheat smut, and potato blight.
Toxins—poisons that can be weaponized after extraction from snakes, insects, spiders, marine organisms, plants, bacteria, fungi, and animals. An example of a toxin is ricin, which is derived from the seed of the castor bean.

Some of these biological agents have properties that would make them more likely candidates for weaponization, such as their lethality, ability to incapacitate, contagiousness or noncontagiousness, hardiness and stability, and other characteristics. Among the agents deemed likely candidates for biological weapons use are the toxins ricin, staphylococcal enterotoxin B (SEB), botulinum toxin, and T-2 mycotoxin and the infectious agents responsible for anthrax, brucellosis, cholera, pneumonic plague, tularemia, Q fever, smallpox, glanders, Venezuelan equine encephalitis, and viral hemorrhagic fever. Various states at various times have looked into weaponizing dozens of other biological agents in addition.

Defense against biological weapons

Military defense

Most weaponized lethal biological agents are intended to be delivered as aerosols, which would cause infections when breathed by the targeted personnel. For this reason, the most-effective defense against biological weapons is a good protective mask equipped with filters capable of blocking bacteria, viruses, and spores larger than one micron (one micrometre; one-millionth of a metre) in cross section from entry into the wearer’s nasal passages and lungs. Protective overgarments, including boots and gloves, are useful for preventing biological agents from contacting open wounds or breaks in the skin. Also, decontaminants can neutralize biological agents in infected areas after a biological attack.

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Developing and fielding effective biological weapon sensors that can trigger an alarm would allow personnel to don masks before exposure, get into protective overgarments, and go inside, preferably into toxic-free collective protection shelters. Medical teams could then immediately go into action to check and treat those who may have been exposed.

Biological warfare attacks can be made less effective, or ineffective, if the targeted persons have been vaccinated against the specific disease-causing agent used in an attack.

Civil defense

Civil defense against biological weapons has greatly improved since the September 11, 2001, attacks in the United States, but progress does not necessarily equal success. A successful civil defense against major biological attacks requires that significant progress be made in sensors, warning systems, vaccines, medicines, training of responders, and public education as well as in planning of emergency procedures. These aspects of civil defense are described briefly in this section, using as examples certain practices put into effect in the United States since September 11.

The foundation of any civil defense against a biological weapons attack is the medical system that has already been set up to deal with naturally occurring diseases. Special vaccines have been created, tested, and approved to deal with the two most lethal biological agents that can also be most easily weaponized: anthrax and smallpox. For example, the U.S. government has enough smallpox vaccine to vaccinate the entire American population and enough anthrax vaccine to inoculate at least every member of the U.S. military.

Effective vaccines for plague and cholera now exist and have been approved for use, but only small quantities have been produced, far short of what might be needed if large numbers of people were to be infected. Furthermore, in the United States a number of vaccines are still in the Investigational New Drug (IND) category and await further trials before the Federal Drug Administration (FDA) can validate their effectiveness and safety. Included among these are vaccines for Q fever, tularemia, Venezuelan equine encephalitis, viral hemorrhagic fever, and botulism.

At present no effective vaccines exist for preventing infections from glanders, brucellosis, staphylococcal enterotoxin B, ricin, or T-2 mycotoxins—all biological agents that some countries have researched for military use or have weaponized in the past. However, in some cases where vaccines are not yet available, medicines have been developed that help the sick to recover.

Long-term medical research is being conducted to investigate the possibility of developing vaccines and supplements that, when administered, might raise the effectiveness of the recipient’s immune system to protect against the whole spectrum of probable biological warfare agents.

One U.S. civil defense program that might make a difference in a biological emergency is the Strategic National Stockpile program, which has created 50-ton “push packages” of vaccines, medicines, decontamination agents, and emergency medical equipment, which are stored in a dozen locations across the country in preparation for emergencies. Furthermore, every U.S. state has bioterrorism response plans in place, including plans or guidelines for mass vaccinations, triage, and quarantines. The U.S. Centers for Disease Control and Prevention (CDC) has also drafted model legislation on emergency health powers for states to adopt in order to deal with such crises.

A new emergency response system was created in the United States following the September 11 attacks. The National Guard increased the number of its Weapons of Mass Destruction Civil Support Teams, which respond to chemical, biological, radiological, or nuclear weapons attacks—augmenting the police, fire, and medical first responders in the local area of any attacks. In addition, the Department of Homeland Security, working with the Department of Health and Human Services, invested heavily in passive defenses against biological attacks, focusing on such programs as Project BioShield and the Laboratory Response Network. The CDC also embarked on a training program on bioterrorism for thousands of medical lab technicians, and the National Institutes of Health funded new biocontainment research laboratories to further research in vaccines, medicines, and bioforensics.

Sensors to detect the presence of biological agents in the air, in water, or on surfaces are still relatively ineffective, but the aim of research is to create a “detect-to-warn” system that would provide enough time for potential victims to don masks, cover up, and take shelter before they are infected. The current “detect-to-treat” capability is unsatisfactory because responders would be treating many persons already infected. Most current biological detectors are point detectors, which are not capable of giving advance warning after scanning an airborne cloud of particles to discern if those particles contain biological agents of a specific type.

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Biological warfare agents

Defense against biological weapons

Military defense

Civil defense