Biological warfare is the deliberate use of living organisms or their toxic byproducts to cause disease and death in humans, animals, or plants. The agents involved range from bacteria like anthrax to viruses like smallpox to plant-derived poisons like ricin. What separates biological warfare from other forms of attack is that many of these agents can replicate inside their victims, and some can spread from person to person, potentially triggering epidemics that extend far beyond the initial target.
How Biological Weapons Differ From Chemical Ones
The distinction matters because biological and chemical weapons work on fundamentally different timelines and scales. Chemical weapons, such as nerve agents or mustard gas, act within seconds to minutes. They are synthetic toxic substances that poison cells directly, blocking oxygen transport, destroying tissue, or shutting down the nervous system. A chemical attack is immediate and localized.
Biological weapons operate on a delay of days, because the pathogen needs time to infect cells and multiply inside the body. That incubation period makes an attack harder to detect but also harder to attribute. A 1970 comparison of equivalent-scale attacks on an unprepared population estimated that a nuclear weapon would kill roughly 90% of those affected, a chemical weapon about 50%, and a biological weapon would sicken about 50%, though deaths could follow depending on the agent and available medical care. The delayed onset also means that by the time anyone realizes an attack has occurred, exposed people may have already traveled and spread the infection further.
Types of Biological Agents
Biological weapons fall into three broad categories based on what they are made of.
Bacteria are single-celled organisms that invade host tissues and multiply, or produce toxins, or both. Anthrax is the most frequently cited example. Its spores can survive in soil for decades, making it unusually stable and easy to stockpile. Once inside the body, anthrax bacteria produce two toxins: one causes severe tissue swelling, and the other destroys immune cells while triggering a dangerous inflammatory cascade.
Viruses are subcellular organisms that hijack host cells to reproduce, often killing those cells in the process. Smallpox was historically the most feared viral weapon because of its high death rate and person-to-person spread. Other viral threats include Nipah virus and hemorrhagic fever viruses.
Toxins are poisons produced by living organisms but are not themselves alive. They do not reproduce or spread between people, which makes them less dangerous than living pathogens but still extremely potent. Botulinum toxin, produced by a bacterium, is one of the most poisonous substances known. It works by blocking the chemical signals between nerves and muscles, causing paralysis. Ricin, extracted from castor beans, destroys ribosomes, the cellular machinery that builds proteins, effectively shutting down a cell’s ability to function.
How the CDC Classifies Threat Agents
The Centers for Disease Control and Prevention ranks biological threat agents into three tiers based on how easily they spread, how stable they are, and how much damage they could cause.
- Category A agents pose the highest risk. They include anthrax, smallpox, plague, and botulinum toxin. These can be widely disseminated, cause high death rates, and would require major public health mobilization.
- Category B agents are moderately easy to spread and cause lower death rates but still significant illness. Examples include certain forms of food poisoning agents and waterborne pathogens.
- Category C agents are emerging threats that could be engineered for mass exposure in the future. This category includes Nipah virus, hantavirus, yellow fever, and multidrug-resistant tuberculosis. These agents appeal to potential attackers because they are relatively cheap, accessible, and harder to detect.
A Long and Disturbing History
Biological warfare is far older than modern microbiology. In 1155, Emperor Barbarossa poisoned water wells with human corpses during a siege in Tortona, Italy. In 1346, Mongol forces catapulted plague-infected bodies over the walls of Caffa on the Crimean Peninsula, likely contributing to the spread of the Black Death into Europe. In 1763, British forces distributed blankets from smallpox patients to Native Americans.
The scale grew dramatically in the 20th century. During World War I, Germany ran covert programs using anthrax and glanders to infect enemy livestock, though with limited success. Japan’s program was far more aggressive: during World War II, the Japanese military poisoned more than 1,000 water wells in Chinese villages to study cholera and typhus outbreaks in civilian populations.
The Cold War brought industrial-scale production. The Soviet Union maintained a massive secret bioweapons program despite having signed treaties banning them. In 1979, an accident at a Soviet bioweapons facility in Sverdlovsk (now Ekaterinburg) released anthrax spores into the surrounding area when a clogged air filter was removed but not replaced between shifts. At least 66 people died. Soviet authorities covered up the incident for over a decade, initially blaming contaminated meat.
International Law and the Weapons Ban
The Biological Weapons Convention, opened for signature in 1972, is the primary international treaty governing these weapons. It now has 189 member states. The treaty’s core provisions are straightforward: states commit to never developing, producing, stockpiling, or retaining biological weapons under any circumstances. They must destroy existing stockpiles or divert them to peaceful purposes. They also agree not to transfer biological weapons technology or assist anyone else in acquiring it. Each member state is required to pass domestic laws enforcing these prohibitions within its own territory.
The convention’s major weakness is verification. Unlike nuclear weapons treaties, it has no formal inspection regime, which means compliance depends largely on good faith and intelligence gathering.
The Dual-Use Problem
One of the trickiest aspects of biological threats is that the same research needed to defend against disease can also be misused to cause it. The National Institutes of Health defines “dual-use research of concern” as life sciences work that could reasonably be expected to provide knowledge, products, or technologies directly misapplied to threaten public health, agriculture, or national security.
This is not a theoretical worry. Studying how a virus becomes more transmissible is essential for pandemic preparedness, but the same findings could help someone engineer a more dangerous pathogen. U.S. policy attempts to balance these risks through institutional oversight that preserves the benefits of research while minimizing the chance of misuse. In practice, this means review committees evaluate proposed experiments before they begin, assessing whether the potential for harm outweighs the scientific value.
Detection and Defense
Defending against biological attacks is harder than defending against conventional or even chemical weapons, largely because of the incubation delay. By the time patients show symptoms, days may have passed since exposure. Modern biodefense strategies focus on closing that gap through rapid diagnostic tools, especially point-of-care tests that can identify unusual pathogens at the bedside rather than requiring samples to be sent to a central lab.
Governments also maintain strategic stockpiles. The United States, for example, has procured millions of doses of next-generation smallpox vaccine and tens of thousands of doses of an antibody treatment targeting anthrax toxin. An ambitious national goal calls for being able to develop, manufacture, test, and license a vaccine against a novel biological threat within six months of identifying it.
At the international level, the World Health Organization coordinates responses to suspected deliberate biological events under the International Health Regulations. WHO works across sectors, including with security partners, to prepare for and respond to health security risks regardless of their origin, whether a natural outbreak, a lab accident, or a deliberate attack. The challenge is that many of the same surveillance systems designed to catch natural epidemics are the ones that would first detect an act of biological warfare, which means investment in routine public health infrastructure is itself a form of biodefense.