Bioterrorism agents are separated into three categories, depending on how easily they can be spread and the severity of illness or death they cause. Category A agents are considered the highest risk and Category C agents are those that are considered emerging threats for disease.

Category A

These high-priority agents include organisms or toxins that pose the highest risk to the public and national security because they:

• Can be easily spread or transmitted from person to person
• Result in high death rates and have the potential for major public health impact
• Could cause public panic and social disruption
• Require special action for public health preparedness.

Category B

These agents are the second highest priority because they:

• Are moderately easy to spread
• Result in moderate illness rates and low death rates
• Require specific enhancements of CDC's laboratory capacity and enhanced disease monitoring.

Category C

These third highest priority agents include emerging pathogens that could be engineered for mass spread in the future because they:

• Are easily available
• Are easily produced and spread
• Have potential for high morbidity and mortality rates and major health impact.

References:
Centers for Disease Prevention and Control, Emergency Preparedness and Response, Emergency Response Preparedness, http://www.bt.cdc.gov/bioterrorism/, Retrieved Nov-17-2009