There is a heightened threat of biological weapons being used for biological warfare or bioterrorism. Many of the microorganisms and toxins that may be used as such biological weapons can easily be acquired and mass produced. Dissemination of aerosols of these biological agents can produce mass casualties. If used by a terrorist they may overwhelm our current public health system. Some biological agents, such as Bacillus anthracis (anthrax) and botulinum toxin, are considered far more likely than others to be used as biological weapons; smallpox virus was apparently produced in mass quantities by the former Soviet Union and may also be a serious threat. The release of such agents could go undetected for several hours or days and would be followed by mass illnesses and a first line of response by the public health community. Rapid epidemiological investigation to identify the nature of the disease outbreak would be critical for limiting casualties. For many, but not all, biological agents there are medical treatments that can greatly lower the mortality rate. There currently are, however, insufficient supplies of medicinals and trained personnel to cope with a massive bioterrorist or biological warfare use of biological weapons. Increasing our preparedness is critical.