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Cross-reactive cell-mediated immunity: An immune response that does not involve antibodies but rather involves the activation of macrophages, natural killer cells (NK), antigen-specific cytotoxic T-lymphocytes, and the release of various cytokines in response to an antigen (, accessed November 5, 2009).

Dilution: A method of obtaining a pure culture of bacteria or virus by subculturing from the highest dilution in which the organism is demonstrably present (, accessed November 5, 2009).

Distributive justice: Benefits and burdens imposed on the population when the emergency response measures and mitigations are shared equitably and fairly. Endemic: The constant presence of a disease or infectious agent within a given geographic area; it may also refer to the usual prevalence of a given disease within such area.

Enzootic: A disease (can be either low or high morbidity) that is endemic in an animal community.

Epidemic: The occurrence in a community or region of cases of an illness (or outbreak) with a frequency clearly in excess of normal expectancy. Epidemiology: The branch of science that deals with the incidence, distribution, and control of disease in a population; the sum of the factors controlling the presence or abundance of a disease or pathogen.

Epitopes: The surface portion of an antigen capable of eliciting an immune response and of combining with the antibody produced to counter that response (, accessed November 5, 2009).

Epi-X: Centers for Disease Control and Prevention’s Web-based communications solution for public health professionals. Through Epi-X, CDC officials, state and local health departments, poison control centers, and other public health professionals can access and share preliminary health surveillance information— quickly and securely. Users can also be actively notified of breaking health events as they occur. For more information, see (accessed November 5, 2009).

Founder effect: Changes in gene frequencies that usually accompany starting a new population from a small number of individuals. The newly founded population is likely to have quite different gene frequencies than the source population because

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