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• Epidemiology Terms

- I -

• Immunity, Active

Resistance developed in response to stimulus by an antigen (infecting agent or vaccine) and usually characterized by the presence of antibody produced by the host.



• Immunity, Herd

The resistance of a group to invasion and spread of an infectious agent, based on the resistance to infection of a high proportion of individual members of the group. The resistance is a product of the number susceptible and the probability that those who



• Immunity, Passive

Immunity conferred by an antibody produced in another host and acquired naturally by an infant from its mother or artificially by administration of an antibody-containing preparation (antiserum or immune globulin).



• Incidence Rate

A measure of the frequency with which an event, such as a new case of illness, occurs in a population over a period of time. The denominator is the population at risk; the numerator is the number of new cases occurring during a given time period.



• Incubation Period

A period of subclinical or inapparent pathologic changes following exposure, ending with the onset of symptoms of infectious disease.



• Independent Variable

An exposure, risk factor, or other characteristic being observed or measured that is hypothesized to influence an event or manifestation (the dependent variable).



• Indirect Transmission

The transmission of an agent carried from a reservoir to a susceptible host by suspended air particles or by animate (vector) or inanimate (vehicle) intermediaries.



• Individual Data

Data that have not been put into a frequency distribution or rank ordered.



• Infectivity

The proportion of persons exposed to a causative agent who become infected by an infectious disease.



• Inference, Statistical

In statistics, the development of generalizations from sample data, usually with calculated degrees of uncertainty.



• Interquartile Range

The central portion of a distribution, calculated as the difference between the third quartile and the first quartile; this range includes about one-half of the observations in the set, leaving one-quarter of the observations on each side.