Biological agents, also known as biological weapons or bioweapons, are pathogens used as weapons. 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.

Some 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 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 is an international treaty banning the development, use or stockpiling of biological weapons; as of March 2021, there were 183 states parties to the treaty. Bio-agents are, however, widely studied for both defensive and medical research purposes under various biosafety levels and within biocontainment facilities throughout the world.

The former United States biological weapons 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 US 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 US. 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, and viral hemorrhagic fevers.

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.