INCUBATION PERIOD AND COMMUNICABILITY
The incubation period is the time between exposure to the organism and appearance of the first symptoms of the disease. Generally, organisms that multiply rapidly and produce local infections, such as gonorrhea and influenza, are associated with short incubation pe-riods (eg, 2 – 4 days). Diseases such as typhoid fever, which depend on hematogenous spread and multiplication of the organism in distant target organs to produce symptoms, often have longer incubation periods (eg, 10 days to 3 weeks). Some diseases have even more prolonged incubation periods because of slow passage of the infecting organism to the target organ, as in rabies, or slow growth of the organism, as in tuberculosis. Incuba-tion periods for one agent may also vary widely depending on route of acquisition and in-fecting dose; for example, the incubation period of hepatitis B virus infection may vary from a few weeks to several months.
Communicability of a disease in which the organism is shed in secretions may occur primarily during the incubation period. In other infections, the disease course is short but the organisms can be excreted from the host for extended periods. In yet other cases, the symptoms are related to host immune response rather than the organism’s action, and thus the disease process may extend far beyond the period in which the etiologic agent can be isolated or spread. Some viruses can integrate into the host genome or survive by replicat-ing very slowly in the presence of an immune response. Such dormancy or latency is ex-emplified by the herpesviruses, and the organism may emerge long after the original infection and potentially infect others.
The inherent infectivity and virulence of a microorganism are also important deter-minants of attack rates of disease in a community. In general, organisms of high infectivity spread more easily and those of greater virulence are more likely to cause disease than subclinical infection. The infecting dose of an organism also varies with different organisms and thus influences the chance of infection and development of disease.