Pathogens of Concern
Smallpox was eradicated in 1980, yet it remains one of the highest-priority biosecurity threats. Its high transmissibility, historically high case fatality, limited population immunity, and the need to rapidly deploy stockpiled countermeasures mean that reintroduction would be catastrophic. Not all pathogens pose equal biosecurity concern. Classification systems prioritize threats based on lethality, transmissibility, countermeasure availability, and historical evidence, without implying that any agent can be readily weaponized.
- Understand the historical CDC Category A, B, and C preparedness categories and distinguish them from current regulatory lists.
- Recognize characteristics that make pathogens high-consequence threats.
- Identify specific select agents on federal oversight lists.
- Evaluate historical use of biological agents in warfare and terrorism.
This chapter discusses biosecurity risks at a conceptual level appropriate for education and policy analysis. Consistent with responsible information practices:
- Omitted: Actionable protocols, specific synthesis routes, exact pathogen sequences
- Included: Risk frameworks, governance mechanisms, policy recommendations
For detailed biosafety protocols, consult your Institutional Biosafety Committee and relevant regulatory guidance.
Introduction
Thousands of pathogens exist that can infect humans. Only a subset pose significant biosecurity concern. Threat assessment frameworks classify biological agents by characteristics that elevate them to high-concern status, and federal select agent lists impose enhanced security on the most dangerous.
The CDC’s biological threat agent categorization system (Categories A, B, C) provides one framework. The Federal Select Agent Program provides another, with regulatory teeth. Both systems exist because not all pathogens warrant the same level of security, oversight, or public health preparedness investment.
Understanding which pathogens matter most for biosecurity and why helps direct defensive resources appropriately.
Historical CDC Biological Threat Agent Categories
CDC developed the Category A, B, and C framework for early-2000s bioterrorism preparedness. It remains useful historical context but is not the current federal regulatory classification. Current possession, use, and transfer requirements are governed through the Federal Select Agent Program.
Category A: Highest Priority Agents
Category A agents pose the highest risk to national security and public health. Characteristics include:
Can be easily disseminated or transmitted from person to person. Cause high mortality, high public health impact potential. May cause public panic and social disruption. Require special action for public health preparedness.
Anthrax (Bacillus anthracis): Spore-forming bacterium causing severe, often fatal illness. Historical use in biological warfare programs and 2001 Amerithrax attacks. Environmental persistence through spores makes it weaponization candidate. Cutaneous, inhalational, and gastrointestinal forms with inhalational having highest mortality without treatment. Vaccine and antibiotics available but inhalational anthrax requires immediate treatment for survival.
Plague (Yersinia pestis): Causes bubonic, septicemic, and pneumonic forms. Pneumonic plague is transmissible person-to-person through respiratory droplets, making it concerning bioterrorism agent. Historical use in medieval biological warfare (catapulting infected corpses) and Unit 731 attacks in China. High mortality if untreated but responds to antibiotics when caught early.
Smallpox (variola virus): Only Category A agent declared eradicated (1980). Highly contagious, historically associated with about 30% case fatality, and still a major preparedness concern because routine vaccination stopped decades ago. Vaccines and FDA-approved antivirals are held for response, while known variola stocks remain restricted to two WHO-authorized repositories (FDA Smallpox Preparedness).
Tularemia (Francisella tularensis): A highly infectious bacterium that can cause severe disease. It is not ordinarily transmitted person to person, but its environmental and occupational exposure risks require strong laboratory and public-health controls. Treatable with antibiotics when recognized promptly.
Botulism (Clostridium botulinum toxin): One of the most lethal toxins known. Prevents muscle contraction through neurotoxin action. Studied extensively by bioweapons programs including Japan’s Unit 731 and investigated by Aum Shinrikyo. Not contagious but toxin can contaminate food/water. Antitoxin available but requires early administration.
Viral Hemorrhagic Fevers (VHFs): Family of viruses causing severe, often fatal illness with vascular damage and bleeding. Category A VHFs include:
- Ebola and Marburg: Fruit bat reservoirs, up to 90% case fatality in some outbreaks. Person-to-person transmission occurs through direct contact with infected bodily fluids. FDA-approved monoclonal treatments exist for Zaire ebolavirus; no approved treatment exists for Marburg virus disease (FDA).
- Lassa fever: Endemic to West Africa, rodent reservoir, ~1% case fatality but high case numbers.
- Argentine, Bolivian, Venezuelan, and Brazilian hemorrhagic fevers: Rodent-borne arenaviruses with regional endemicity.
Category B: Second-Highest Priority
Category B agents are moderately easy to disseminate, cause moderate morbidity and low mortality, require enhanced diagnostic capacity and disease surveillance. Examples include:
Brucellosis (Brucella species): Causes chronic debilitating illness. Studied by bioweapons programs but limited weaponization. Zoonotic from livestock.
Q Fever (Coxiella burnetii): Highly infectious, low mortality, chronic fatigue syndrome in some cases. Environmental persistence. Studied for weaponization.
Ricin toxin: A plant-derived protein toxin for which no licensed antidote is available. Historical criminal use and bioterrorism plots have made it a regulated concern.
Food/waterborne pathogens: Salmonella, Shigella, E. coli O157:H7, Vibrio cholerae. Demonstrated in 1984 Rajneeshee attack (Salmonella). Moderate public health impact but feasible for non-state actors.
Category C: Emerging Threats
Category C was used by CDC as a forward-looking grouping for emerging pathogens that could pose serious public-health consequences if their epidemiology or accessibility changed. The category is not a forecast that any agent can be engineered or disseminated effectively. Examples include:
Nipah virus: Bat-borne paramyxovirus, up to 75% case fatality, limited person-to-person transmission. Endemic to Southeast Asia. Biosafety Level 4 pathogen with pandemic potential if transmissibility increased.
Hantaviruses: Rodent-borne viruses causing hemorrhagic fever with renal syndrome or hantavirus pulmonary syndrome. Most hantaviruses are not transmitted person to person; Andes virus has caused rare person-to-person transmission (CDC).
Tick-borne encephalitis viruses: Flaviviruses that can cause neurological illness and are endemic in parts of Europe and Asia.
Multidrug-resistant tuberculosis: A major public-health burden requiring surveillance, infection control, and effective treatment programs.
Federal Select Agent Program
Select Agent List
Currently 63 biological agents and toxins on select agent list. Organized into:
- HHS select agents: Human pathogens and toxins
- USDA select agents: Plant and animal pathogens
- Overlap select agents: Affect both humans and animals
Inclusion criteria based on effect on human health, effect on animal or plant health, pathogenicity, virulence, dissemination potential.
Regulatory Requirements
Entities possessing select agents must:
- Register with CDC/APHIS
- Implement biosafety and biosecurity plans
- Restrict regulated access through Security Risk Assessments; Tier 1 agents also require enhanced pre-access and ongoing suitability measures (FSAP)
- Maintain detailed inventory and transfer records
- Report theft, loss, or release
- Submit to inspections
Violations can result in civil and criminal penalties, suspension or revocation of registration, remedial actions.
Balancing Research and Security
Select agent regulations aim to balance legitimate research needs against misuse potential. Tension exists between open science culture and security requirements.
Concerns about regulations stifling research led to periodic list reviews and exemptions for attenuated strains or select toxins below concentration thresholds. But 2001 anthrax attacks demonstrated insider threat risk, justifying strict oversight.
Characteristics of High-Consequence Pathogens
Case Fatality Rate
Highly lethal pathogens cause greater panic and social disruption. Smallpox (~30%), pneumonic plague (nearly 100% if untreated), Ebola (up to 90% in outbreaks), inhalational anthrax (~85% if untreated) all have high case fatality.
But lethality alone does not determine biosecurity concern. Rabies has nearly 100% case fatality but is not Category A due to transmission requirements.
Transmissibility
Person-to-person transmission amplifies outbreak potential. Smallpox is contagious. Pneumonic plague spreads through respiratory droplets. VHFs transmit through bodily fluid contact.
Environmental persistence (anthrax spores) or vector-borne transmission (plague via fleas) provides alternative dissemination routes.
Lack of Medical Countermeasures
Pathogens without effective vaccines or treatments pose greater threat. Smallpox eradication led to vaccination discontinuation, leaving populations vulnerable. Ebola and Marburg have limited treatment options.
Availability of antibiotics reduces bacterial threat (plague, tularemia, anthrax treatable if caught early) but requires rapid diagnosis and treatment initiation.
Misuse Potential
Risk assessment considers whether an agent or toxin has characteristics relevant to deliberate misuse, together with actor access, expertise, controls, and available countermeasures.
Pathogens requiring complex cultivation, specific environmental conditions, or live vectors are harder to weaponize effectively. Aum Shinrikyo’s biological weapons failures demonstrated technical barriers.
Historical Use
Agents with documented use in biowarfare or bioterrorism receive heightened scrutiny. Anthrax (2001 letters, past bioweapons programs), plague (Unit 731, medieval warfare), tularemia and botulinum toxin (multiple bioweapons programs) all have weaponization history.
Historical precedent informs risk assessment even when technical barriers exist.
This chapter is part of The Biosecurity Handbook.