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Biological Weapons

By James Madsen, MD, MPH, Adjunct Associate Professor of Preventive Medicine and Biometrics;Chief, Consultant Branch, Chemical Casualty Care Division, Uniformed Services University of the Health Sciences;US Army Medical Research Institute of Chemical Defense, Aberdeen Proving Ground South, MD

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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. Biological weapons are thought by some to be an ideal weapon 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 ( 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

Variola virus, causing variola major (classic smallpox)

Francisella tularensis, causing tularemia

Viral-hemorrhagic-fever (VHF) viruses

  • Arenaviruses, causing Lassa fever and New World VHFs (Machupo, Junin, Guanarito, and Sabia hemorrhagic fevers)

  • Bunyaviridae, causing Crimean Congo hemorrhagic fever and Rift Valley fever

  • Filoviridae, causing Ebola virus disease and Marburg virus disease

  • Flaviviridae, causing yellow fever, Omsk hemorrhagic fever, and Kyasanur Forest disease

B: 2nd highest priority

Brucella species, causing brucellosis

Epsilon toxin of Clostridium perfringens

Salmonella sp, Escherichia coli0157:H7, and Shigella, causing food poisoning

Burkholderia mallei, causing glanders

Burkholderia pseudomallei, causing melioidosis

Chlamydia psittaci, causing psittacosis

Coxiella burnetii, causing Q fever

Ricin toxin from Ricinus communis

Staphylococcal enterotoxin B

Rickettsia prowazekii, causing typhus fever

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

Vibrio cholerae, Cryptosporidium parvum, and other agents, causing waterborne diseases

C: 3rd highest priority

Nipah virus, hantavirus, SARS coronavirus, and influenza viruses capable of causing pandemic influenza

Other agents associated with emerging infectious diseases

CDC = US Centers for Disease Control and Prevention; SARS = severe acute respiratory syndrome.


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 disease caused by high-risk biological weapons are discussed elsewhere in The Manual: Anthrax (see Anthrax), plague (see Plague), smallpox (see Smallpox), tularemia (see Tularemia), and viral hemorrhagic fevers (see Hemorrhagic Fevers Overview).

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 US Government.

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