In reality, a large number of biological agents exist that could pose a public health threat. However in order to assist in the public health planning and preparedness efforts, the CDC screened and categorized all agents based on four major factors to determine which agents posed the greatest threat to civilian populations. According to the article "Public Health Assessment of Potential Biological Terrorism Agents" [CDC Emerging Infectious Disease, February 2002], the four factors include:

1. Public health impact;
2. Delivery potential to large populations;
3. Public perception of the agent (public fear and potential for civil disruption); and
4. Special public health preparedness needs required to respond to the agent.

Biological agents were identified using these four factors and then re-evaluated based on a number of additional parameters. Through this process agents were categorized as one of three agent types: Type A, Type B, and Type C.

Type A Agents
Type A agents include organisms that pose a risk to national security because they:

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

Due to their high risk, the CDC has determined that the public health preparedness efforts required to plan, prepare, and respond to a Type A agents include improved surveillance and laboratory diagnosis as well as the stockpiling of specific medications.

Type B Agents
Type B agents have the second highest priority in terms of impact and preparedness requirements. These agents characteristically

• Are moderately easy to spread;
• Result in moderate morbidity rates and low mortality rates; and
• Require specific enhancements of CDC's diagnostic capacity and enhanced disease surveillance.

In comparison to Type A agents, Type B agents generally have fewer special public health preparedness requirements. Some improvement in public health and medical awareness, surveillance, or laboratory diagnosis may be required, however there are limited additional requirements for stockpiled medications.

Type C Agents
The final group of agents having the third highest priority includes emerging disease producing agents that do not currently pose a bioterrorism threat to public health. However, public health officials believe Type C agents could be engineered for mass transmission in the future due to:

• Availability;
• Ease of production and transmission; and
• Potential for high morbidity and mortality rates and major health impact.

Due to their minimal threat, the CDC has determined that public health preparedness needs for Type C agents will be addressed through overall bioterrorism preparedness and public health infrastructure improvements aimed at detecting emerging infectious diseases.