the form of specific disease in specific subgroups of the human population (Rubin, 1987), whereas many particles of a chemical agent would be necessary to provoke an effect—depending on the chemical agent and the mode of action. In the case of microbial agents, an ingested, inhaled, or absorbed microorganism can multiply within the body of a susceptible human, to produce sufficient microorganisms in vivo that illness can result. However, the ability of the microorganisms to multiply does not imply that the exposure to a single organism will always produce illness, because the organism could be killed by defense processes—for example, the acidity of the gastrointestinal tract or the action of the immune system—before it can reproduce in sufficient numbers to cause an effect. That said, a single organism has the biological potential to produce an effect if a sufficient number of progeny are produced and survive. The ability of one organism to produce such an effect is strongly dependent on the susceptibility of the host, although susceptibility varies extensively in the human population, which consists of the young, the old, the healthy, and the immunocompromised.
The third difference is that the microbial exposure of one individual also can have a subsequent effect in the broader population (including persons who are not exposed directly). Some diseases are transmissible from person to person and can be spread even via asymptomatic individuals. Therefore, interpersonal contact can cause others to become ill. The degree of secondary spread depends on the organism’s infectivity and its excretion pattern and on the behavior of infected persons.