This chapter defines a biological threat agent as any living microorganism, or the toxic product derived from a living organism, that may cause disease or adverse physiological response in animals, humans, and plants. Human activities that alter natural ecosystems likely also affect the transmission cycles and dynamics of naturally occurring biological agents in the environment. Naturally occurring disease outbreaks, epidemics, and pandemics have significantly shaped human history, and the psychological terror associated with such historic events are most certainly associated with the contemporary perspective and effectiveness of biological threats. The occurrence of an infectious disease agent in a specific geographic region or human population may be a significant indication for social, demographic, and economic factors. The increasing threats to human public health, as well as to animal health from the intentional and natural exposure to disease-causing pathogens can be directly related to our vulnerabilities as a result of a wide range of environmental, societal, cultural, and economic factors.