A biological agent (also called bio-agent, biological threat agent, biological warfare agent, biological weapon, or bioweapon) is a bacterium, virus, protozoan, parasite, fungus, or toxin 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 1972 Biological Weapons Convention (BWC) is an international treaty banning the development, use or stockpiling of biological weapons; as of March 2021, there were 183 States Parties to the BWC. Bio-agents are, however, widely studied for both defensive and medical research purposes under various biosafety levels and within biocontainment facilities throughout the world. Classifications Operational 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). Legal 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". Regulatory 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