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. The toxicity of an agent reflects the relative severity of illness or incapacitation produced by a toxin. This reflects the capability of an infectious agent to cause disease in a susceptible host. The time between exposure and the appearance of symptoms is known as the incubation period.
Lethality reflects the relative ease with which an agent causes death in a susceptible population.
The viability of an agent is affected by various environmental factors, including temperature, relative humidity, atmospheric pollution, and sunlight. Taxonomic classification of biological agents is important to the medical services in terms of detection, identification, prophylaxis, and treatment. 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.