The categories of bioterror agents are distinguished by assessments of the agents' ease of transmission, severity of morbidity/mortality, and the potential risk they pose to national security. Category A agents, which pose the greatest risk, contain organisms that pose the most risk to national security because they can be easily disseminated or transmitted from person to person, resulting in high mortality rates and a major impact on public health. Category A agents include anthrax, smallpox, tularemia, plague, and a variety of organisms that cause viral hemorrhagic fever. Category B agents are moderately easy to disseminate and result in moderate morbidity and low mortality. Still, they would cause severe disease outbreaks, disabling many people and creating panic and chaos in the general population. Category B contains 47 potential agents, some of which are toxins produced by bacteria. A few of the most potent agents are identified and described, along with those that have the greatest potential for use by a terrorist group. Category C agents consist of those organisms that could be developed or engineered for mass dissemination in the future. This potential is based on the organism's availability, ease of production and distribution, and mortality or health impact it could deliver. One of the major agent groups in Category C is the hantaviruses, which belong to the bunyaviridae family of VHF viruses. Their widespread presence in rodents, particularly deer mice in the Southwest, makes these viruses a potential agent for terrorists.