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Category “A” Biological Agents are high-priority agents which pose the greatest security and health threat in a population. They are the primary most dangerous biological agents with the highest research preference due to their ease of dissemination, high morbidity, and high mortality. Agents in this category have the potential to cause mass public fear and societal disorder. Category “A” biological agents requires special public health preparedness and action, and they can be easily transmitted from person-to-person after being discharged in a defined animal or human population. They have the utmost potential for adverse security and public health impact which may result in multitudes of fatalities. Samples suspected to contain biological agents in Category “A” are analyzed in Biological Safety Level-4 (BL4) laboratories. BL4 laboratories are designed to handle and contain infectious specimens and pathogens that are suspected to be potentially dangerous to the general public.

Examples of biological agents in Category “A” include:

  • Bacillus anthracis – that cause anthrax.
  • Variola major(Smallpox) virus – that cause smallpox.
  • Yersinia pestis – that cause plague.
  • Francisella tularensis – that cause tularemia.
  • Haemorrhagic fever viruses (e.g. Ebola virus, Marburg, Lassa fever, Machupo, and Hantaviruses) – that cause viral haemorrhagic fevers.
  • Clostridium botulinum – that cause botulism.


Category “B” Biological Agents are the second highest priority biological agents that have lesser public alertness than agents in Category “A”. They are moderately easy to disseminate. Biological agents of concern for water and food safety are included in Category “B”. Agents in Category “B” cause less morbidity and mortality, and they are mostly implicated in outbreaks of food-borne and water-borne diseases.

Examples of biological agents in Category “B” include:  

  • Staphylococcal enterotoxin B – that cause food poisoning.
  • Brucella abortus – that causes brucellosis.
  • Chlamydia psittaci – that causes psittacosis.
  • Rickettsia prowazekii ­– that causes typhus fever.
  • Coxiella burnetti – that causes Q fever.
  • Burkholderia mallei – that causes glanders.
  • Burkholderia pseudomallei – that causes melioidosis.
  • Ricin toxin from castor beans (Ricinus communis).
  • Epsilon toxin of Clostridium perfringens
  • Food safety threat pathogens (e.g. Salmonella, Escherichia coli O157:H7, Shigella).
  • Water safety threat pathogens (e.g. Vibrio cholerae, Cryptosporidium parvum).
  • Alphaviruses – that causes viral encephalitis (e.g. Western Equine Encephalitis, Venezuelan equine encephalitis, Eastern Equine Encephalitis).     
  • Abrin toxin from Rosary peas (Abrus precatorius). 


Category “C” Biological Agentsare the third highest priority biological agents that present no immediate security and public threat because they are still evolving microbes. They are biological agents which at the moment do not present a high risk of bioterrorism to the general public but on the other hand could materialize as potential biologic threat in the future. Agents in Category “C” are referred to as emerging pathogens with a potential for high morbidity and mortality, and information regarding their epidemiology and pathogenicity is still arcane because they are emerging diseases. Though they have not been previously used in bioterrorism as some agents in Categories “A and B”, biological agents in Category “C” can be easily accessed due to their availability and ease of production and dissemination; and these can be genetically engineered to produce potent biologic material for mass destruction in human or animal population.

Examples of biological agents in Category “C” include:  

  • Severe acute respiratory syndrome (SARS).
  • Hantaviruses.
  • Influenza A (H5N1).
  • Nipah virus. 
  • Multidrug resistant tuberculosis.
  • Mycotoxin-producing fungi.


  • Nerve agents: These are highly poisonous chemicals that work by preventing the nervous system from working properly.
  • Biotoxins: These are poisons that come from plants or animals.
  • Blister agents/vesicants—chemicals that severely blister the eyes, respiratory tract, and skin on contact.
  • Blood agents: They are poisons that affect the body by being absorbed into the blood.
  • Caustics (acids): These are chemicals that burn or corrode people’s skin, eyes, and mucus membranes (lining of the nose, mouth, throat, and lungs) on contact.
  • Choking/lung/pulmonary agents: These are chemicals that cause severe irritation or swelling of the respiratory tract (lining of the nose and throat, lungs).
  • Incapacitating agents: These are drugs that make people unable to think clearly or that cause an altered state of consciousness (possibly unconsciousness).
  • Long-acting anticoagulants: These are poisons that prevent blood from clotting properly, which can lead to uncontrolled bleeding.
  • Metals: These areagents that consist of metallic poisons.
  • Organic solvents: These are agents that damage the tissues of living things by dissolving fats and oils.
  • Riot control agents/tear gas: These agents are highly irritating agents normally used by law enforcement agents such as the police for crowd control or by individuals for protection (for example, mace).
  • Toxic alcohols: These are poisonous alcohols that can damage the heart, kidneys, and nervous system
  • Vomiting agents: These are chemicals that cause nausea and vomiting.


Aschengrau A and Seage G.R (2013). Essentials of Epidemiology in Public Health. Third edition. Jones and Bartleh Learning,

Gordis L (2013). Epidemiology. Fifth edition. Saunders Publishers, USA.

Nelson K.E and Williams C (2013). Infectious Disease Epidemiology: Theory and Practice. Third edition. Jones and Bartleh Learning. 

Riedel S (2004). Biological warfare and bioterrorism: a historical overview. BUMC Proceedings, 17:400-406. 

Rothman K.J, Greenland S and Lash T.L (2011). Modern Epidemiology. Third edition. Lippincott Williams and Wilkins, Philadelphia, PA, USA.

Schneider M.J (2011). Introduction to Public Health. Third edition. Jones and Bartlett Publishers, Sudbury, Massachusetts, USA.

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