Home > Biological Agents

Overview Biological Agents


a. Characteristics. Intrinsic features of biological agents which influence their potential for use as weapons include: infectivity; virulence; toxicity; pathogenicity; incubation period; transmissibility; lethality; and stability. Unique to many of these agents, and distinctive from their chemical counterparts, is the ability to multiply in the body over time and actually increase their effect.

b. Infectivity. The infectivity of an agent reflects the relative ease with which microorganisms establish themselves in a host species. Pathogens with high infectivity cause disease with relatively few organisms, while those with low infectivity require a larger number. High infectivity does not necessarily mean that the symptoms and signs of disease appear more quickly, nor that the illness is more severe.

c. Virulence. The virulence of an agent reflects the relative severity of disease produced by that agent. Different microorganisms and different strains of the same microorganism may cause diseases of different severity.

d. Toxicity. The toxicity of an agent reflects the relative severity of illness or incapacitation produced by a toxin.

e. Pathogenicity. This reflects the capability of an infectious agent to cause disease in a susceptible host.

f. Incubation Period. A sufficient number of microorganisms or quantity of toxin must penetrate the body to initiate infection (the infective dose), or intoxication (the intoxicating dose). Infectious agents must then multiply (replicate) to produce disease. The time between exposure and the appearance of symptoms is known as the incubation period. This is governed by many variables, including: the initial dose; virulence; route of entry; rate of replication; and host immunological factors.

g. Transmissibility. Some biological agents can be transmitted from person-to-person directly. Indirect transmission (for example, via arthropod vectors) may be a significant means of spread as well. In the context of BW casualty management, the relative ease with which an agent is passed from person-to-person (that is, its transmissibility) constitutes the principal concern.

h. Lethality. Lethality reflects the relative ease with which an agent causes death in a susceptible population.

i. Stability. The viability of an agent is affected by various environmental factors, including temperature, relative humidity, atmospheric pollution, and sunlight. A quantitative measure of stability is an agent's decay rate (for example, "aerosol decay rate").

j. Additional Factors. Additional factors which may influence the suitability of a microorganism or toxin as a biological weapon include: ease of production; stability when stored or transported; and ease of dissemination.


a. Medical. (See Annexes A and B.) Taxonomic classification of biological agents is important to the medical services in terms of detection, identification, prophylaxis, and treatment. Biological agents which may be used as weapons can be classified as follows:

(1) Bacteria. Bacteria are small free-living organisms, most of which may be grown on solid or liquid culture media. The organisms have a structure consisting of nuclear material, cytoplasm, and cell membrane. They reproduce by simple division. The diseases they produce often respond to specific therapy with antibiotics.

(2) Viruses. Viruses are organisms which require living cells in which to replicate. They are therefore intimately dependent upon the cells of the host which they infect. They produce diseases which generally do not respond to antibiotics but which may be responsive to antiviral compounds, of which there are few available, and those that are available are of limited use.

(3) Rickettsiae. Rickettsiae are microorganisms which have characteristics common to both bacteria and viruses. Like bacteria, they possess metabolic enzymes and cell membranes, utilize oxygen, and are susceptible to broad-spectrum antibiotics. They resemble viruses in that they grow only within living cells.

(4) Chlamydia. Chlamydia are obligatory intracellular parasites incapable of generating their own energy source. Like bacteria, they are responsive to broad-spectrum antibiotics. Like viruses, they require living cells for multiplication.

(5) Fungi. Fungi are primitive plants which do not utilize photosynthesis, are capable of anaerobic growth, and draw nutrition from decaying vegetable matter. Most fungi form spores, and free-living forms are found in soil. The spore forms of fungi are operationally significant. Fungal diseases may respond to various antimicrobials.

(6) Toxins. Toxins are poisonous substances produced and derived from living plants, animals, or microorganisms; some toxins may also be produced or altered by chemical means. Toxins may be countered by specific antisera and selected pharmacologic agents.

b. Operational. It may be considered useful to classify biological agents by the effects they produce in an operational context, in order to provide guidance to the field commander on the consequences for continued operational effectiveness. Annex C of this manual provides guidance for such a classification scheme by individual agent. Operational categories should incorporate all recognized variables likely to impact on effectiveness, to include lethality, transmissibility, and persistence.