A biological agent (also called bio-agent, biological threat agent, biological warfare agent, biological weapon, or bioweapon) is a bacterium, virus, protozoan, parasite, or fungus that can be used purposefully as a weapon in bioterrorism or biological warfare (BW). In addition to these living or replicating pathogens, toxins and biotoxins are also included among the bio-agents. More than 1,200 different kinds of potentially weaponizable bio-agents have been described and studied to date.
Biological agents have the ability to adversely affect human health in a variety of ways, ranging from relatively mild allergic reactions to serious medical conditions, including serious injury, as well as serious or permanent disability or even death. Many of these organisms are ubiquitous in the natural environment where they are found in water, soil, plants, or animals. Bio-agents may be amenable to "weaponization" to render them easier to deploy or disseminate. Genetic modification may enhance their incapacitating or lethal properties, or render them impervious to conventional treatments or preventives. Since many bio-agents reproduce rapidly and require minimal resources for propagation, they are also a potential danger in a wide variety of occupational settings.
The 1975 Biological Weapons Convention (BWC) is an international treaty banning the development, use or stockpiling of biological weapons; as of October 2020, there were 183 States Parties to the BWC. Bio-agents are, however, widely studied for both defensive and general medical purposes under various biosafety levels and within biocontainment facilities throughout the world.
The former US biological warfare program (1943-1969) categorized its weaponized anti-personnel bio-agents as either "lethal agents" (Bacillus anthracis, Francisella tularensis, Botulinum toxin) or "incapacitating agents" (Brucella suis, Coxiella burnetii, Venezuelan equine encephalitis virus, Staphylococcal enterotoxin B).
Since 1997, United States law has declared a list of bio-agents designated by the U.S. Department of Health and Human Services or the U.S. Department of Agriculture that have the "potential to pose a severe threat to public health and safety" to be officially defined as "select agents" and possession or transportation of them are tightly controlled as such. Select agents are divided into "HHS select agents and toxins", "USDA select agents and toxins" and "Overlap select agents and toxins".
The U.S. Centers for Disease Control and Prevention (CDC) breaks biological agents into three categories: Category A, Category B, and Category C. Category A agents pose the greatest threat to the U.S. Criteria for being a Category "A" agent include high rates of morbidity and mortality; ease of dissemination and communicability; ability to cause a public panic; and special action required by public health officials to respond. Category A agents include anthrax, botulism, plague, smallpox, tularemia, and viral hemorrhagic fevers.
|Disease||Causative Agent (Military Symbol)|
|Anthrax||Bacillus anthracis (N or TR)|
|Brucellosis (bovine)||Brucella abortus|
|Brucellosis (caprine)||Brucella melitensis (AM or BX)|
|Brucellosis (porcine)||Brucella suis (US, AB or NX)|
|Cholera||Vibrio cholerae (HO)|
|Diphtheria||Corynebacterium diphtheriae (DK)|
|Dysentery (bacterial)||Shigella dysenteriae, some species of Escherichia coli (Y)|
|Glanders||Burkholderia mallei (LA)|
|Listeriosis||Listeria monocytogenes (TQ)|
|Melioidosis||Burkholderia pseudomallei (HI)|
|Plague||Yersinia pestis (LE)|
|Tularemia||Francisella tularensis (SR or JT)|
|Disease||Causative Agent (Military Symbol)|
|Q Fever||Coxiella burnetii (OU)|
|Rocky Mountain spotted fever||Rickettsia rickettsii (RI or UY)|
|Typhus (human)||Rickettsia prowazekii (YE)|
|Typhus (murine)||Rickettsia typhi (AV)|
|Disease||Causative Agent (Military Symbol)||Comments|
|Equine Encephalitis (Eastern)||Eastern equine encephalitis virus (ZX)|
|Equine Encephalitis (Venezuelan)||Venezuelan Equine Encephalomyelitis virus (FX)|
|Equine Encephalitis (Western)||Western equine encephalitis virus (EV)|
|Japanese B encephalitis||Japanese encephalitis virus (AN)|
|Marburg Hemorrhagic Fever (Marburg HF)||Marburg Virus (MARV)||by the Soviet Union|
|Rift Valley fever||Rift Valley fever virus (FA)|
|Smallpox||Variola virus (ZL)|
|Yellow fever||Yellow fever virus (OJ or LU)|
|Toxin||Source of Toxin (Military Symbol)|
|Abrin||Rosary pea (Abrus precatorius)|
|Botulinum toxins (A through G)||Clostridium botulinum bacteria or spores, and several other Clostridial species. (X or XR)|
|Ricin||Castor bean (Ricinus communis) (W or WA)|
|Saxitoxin||Various marine and brackish cyanobacteria, such as Anabaena, Aphanizomenon, Lyngbya, and Cylindrospermopsis (TZ)|
|Staphyloccocal enterotoxin B||Staphylococcus aureus (UC or PG)|
|Tetrodotoxin||Various marine bacteria, including Vibrio alginolyticus, Pseudoalteromonas tetraodonis (PP)|
|Trichothecene mycotoxins||Various species of fungi, including Fusarium, Trichoderma, and Stachybotrys|
|Vector (Military Symbol)||Disease|
|Mosquito (Aedes aegypti) (AP)||Malaria, Dengue fever, Chikungunya, Yellow fever, other Arboviruses|
|Oriental rat flea (Xenopsylla cheopis)||Plague, Murine typhus|
Simulants are organisms or substances which mimic physical or biological properties of real biological agents, without being pathogenic. They are used to study the efficiency of various dissemination techniques or the risks caused by the use of biological agents in bioterrorism. To simulate dispersal, attachment or the penetration depth in human or animal lungs, simulants must have particle sizes, specific weight and surface properties, similar to the simulated biological agent.
The typical size of simulants (1-5 µm) enables it to enter buildings with closed windows and doors and penetrate deep into the lungs. This bears a significant health risk, even if the biological agent is normally not pathogenic.