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Biological Weapons


James M. Madsen

, MD, MPH, U.S. Army Medical Research Institute of Chemical Defense (USAMRICD)

Last full review/revision Feb 2021| Content last modified Feb 2021

Biological warfare is the use of microbiological agents as weapons. Such use is contrary to international law and has rarely taken place during formal warfare in modern history, despite the extensive preparations and stockpiling of biological agents carried out during the 20th century.

(See also Overview of Incidents Involving Mass-Casualty Weapons.)

Biological weapons are thought by some to be ideal weapons for terrorists. These agents may be delivered in secret and have delayed effects, allowing the perpetrator to remain undetected.

The US Centers for Disease Control and Prevention (CDC) has created a priority list of biological agents and toxins (see table CDC High-Priority Biological Agents and Toxins). The highest priority are Category A.

The deliberate use of biological weapons to cause mass casualties would probably entail the use of aerosols. Inhalational anthrax and pneumonic plague are the two diseases most likely to occur under these circumstances.

CDC High-Priority Biological Agents and Toxins*



A: Highest priority

Bacillus anthracis, causing anthrax

Botulinum toxin from Clostridium botulinum, causing botulism

Yersinia pestis, causing plague

Francisella tularensis, causing tularemia

Variola virus, causing variola major (classic smallpox)

Viral hemorrhagic-fever (VHF) viruses

B: 2nd highest priority

Brucella species, causing brucellosis

Epsilon toxin of Clostridium perfringens, causing food poisoning

Salmonella sp, causing food poisoning; Escherichia coli 0157:H7 infection; and Shigella, causing shigellosis

Burkholderia mallei, causing glanders

Burkholderia pseudomallei, causing melioidosis

Chlamydia psittaci, causing psittacosis

Coxiella burnetii, causing Q fever

Ricin toxin from Ricinus communis, causing different symptoms depending on type of exposure

Staphylococcal enterotoxin B, causing staphylococcal food poisoning and other symptoms depending on type of exposure

Rickettsia prowazekii, causing typhus fever

Alphaviruses causing viral encephalitides (for example, Venezuelan, eastern, and western equine encephalitides)

Vibrio cholerae, causing cholera; Cryptosporidium parvum, causing cryptosporidiosis; and other agents, causing other waterborne diseases

C: 3rd highest priority

Nipah virus, hantavirus, coronaviruses, and influenza viruses capable of causing pandemic influenza

Other agents associated with emerging infectious diseases

* An alphabetical listing of bioterrorism agents and diseases can be found at Centers for Disease Control and Prevention: Emergency Preparedness and Response.


Distinguishing the use of a biological weapon from a natural outbreak of disease may be difficult for doctors. Clues to a deliberate rather than a natural origin of a disease outbreak include the following:

  • Cases of diseases not usually seen in that geographic area
  • Unusual distribution of cases among segments of the population
  • Significantly different illness rates between those inside and those outside buildings
  • Separate outbreaks in geographically diverse areas
  • Multiple simultaneous or serial outbreaks of different diseases in the same population
  • Unusual routes of exposure (such as inhalation)
  • A disease that normally affects animals occurring in humans
  • A disease that normally affects animals arising in an area where that animal species is not usually present
  • Unusual severity of disease
  • Unusual strains of infectious agents
  • Failure to respond to standard therapy

The symptoms, diagnosis, and treatment of people with the following diseases caused by high-risk biological weapons are discussed elsewhere in The Manuals:

Doctors may need to isolate people exposed to a biological weapon and place under quarantine people known to have been in contact with an exposed person.


Because of the relatively long incubation periods of diseases caused by biological weapons, people will likely be treated in a hospital. People are given vaccines, antibiotics, or antiviral drugs depending on the specific infectious organism involved. Sometimes people who have been in contact with the affected people are given preventive treatment. For many biological weapons, there is no specific treatment or vaccine.

The views expressed in this article are those of the author and do not reflect the official policy of the Department of Army, Department of Defense, or the U.S. Government.

More Information about Biological Weapons

The following English-language resource may be useful. Please note that The Manual is not responsible for the content of this resource.

  • Centers for Disease Control and Prevention: Emergency Preparedness and Response: An alphabetical listing of bioterrorism agents and diseases

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