A bioterrorism attack is the deliberate release of viruses, bacteria, toxins, or other harmful agents used to cause illness or death in people, animals, or plants. These agents are typically found in nature, but it is possible that they could be mutated or altered to increase their ability to cause disease, make them resistant to current medicines, or increase their ability to be spread into the environment. Biological agents can be spread through the air, water, or food. Terrorists tend to use biological agents because they are extremely difficult to detect and do not cause illness for several hours to several days. Some bioterrorism agents, like the smallpox virus, can be spread from person to person, and some, like anthrax, cannot (CDC 2001, 2011).

Bioterrorism agents comprise bacteria, viruses, fungi, and other microorganisms as well as biotoxins formed by microorganisms, plants, and animals that can kill or debilitate. Because they can reproduce, biological agents have the sole potential to make an environment more dan­gerous over time. If used for aggressive pur­poses, any disease-causing microorganism could become a weapon. For the reasons of warfare, the specific uniqueness of certain agents makes them more likely to be used than others. Some potential warfare agents can make their victims very sick without necessarily killing them. Examples include the microorganisms that cause tularemia, Q fever, and yellow fever. After suffering debilitating illness, victims of these diseases often recover, though not always. Other agents are more likely to be lethal. The bacteria that cause bubonic plague and the virus that causes smallpox can kill large numbers of untreated people. Early management regularly cures plague victims, and smallpox vaccinations before exposure to the virus can prevent the disease.

The Centers for Disease Control (CDC 2001; Rotz et al. 2002) separates bioterrorism agents into three categories, depending on how easily they can be spread and the severity of illness or death they cause: