Introduction to Epidemiologic Terms 1
Agent — A biological, physical or chemical entity capable of causing disease
Analytical Epidemiology — Studies which identify and quantify associations, test
hypotheses, and identify causes. Two common study types are cohort and case-control
study. (compare with descriptive epidemiology)
Antibody — A protein, found in tissue fluids and blood, produced in response to the
stimulus of a specific antigen and capable of combining with that antigen to neutralize or
destroy it.
Antigen — That portion or product of a biological agent capable of stimulating the
formation of specific antibodies.
Attack rate — A specific type of incidence, applied to a narrowly defined population
observed for a limited period of time, such as during an epidemic. (see incidence,
prevalence)
Carrier — A person or animal that harbors a specific infectious agent (but manifests no
clinical disease) and is a source of infection for humans or animals. If the infection
leads to disease, the carrier state may occur during the incubation, convalescent, and
post-convalescent periods. The carrier state may be of short or long duration
(temporary carrier or chronic carrier).
Case — A person in a population or study group identified as having a particular
disease or condition under investigation. A variety of criteria can be used to identify
cases and is referred to as the case definition.
Case-control study — A type of observational analytic study. Enrollment into the study is
based on presence (“case”) or absence (“control”) of disease. Characteristics or
previous exposure to a factor of interest are then compared between cases and controls.
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Comment: This document is only an introduction to epidemiologic terms, not a
comprehensive list, or even a list of the most important terms. This list does not include
terms, that are better described in diagrams, examples, or narratives, than in one or two
sentences (such as 2x2 table, confounding, bias, null hypothesis, sensitivity, specificity,
predictive value, etc).
Epidemiology Terms
Case Fatality Rate — The proportion of cases of a specific condition fatal within a
certain time frame: Number of deaths from a disease/Number of diagnosed cases of
that disease (usually
x 100).
Causality — The relationship between two variables whereby a change in one is
followed by a change in the other. The criteria used to assess the likelihood of the
causal nature of an association are its consistency, specificity, strength, temporal
correctness and coherence (biological plausibility).
Cohort study —A type of observational analytic study. Inclusion in the study is based on
exposure characteristics or membership in a group identified prior to development of
disease, death, or other health-related outcomes. These outcomes are then
subsequently ascertained and compared.
Communicability (period of) — The time during which a person or animal with infectious
disease is a potential source of infection.
Descriptive Epidemiology — Epidemiology concerned with organizing and summarizing
health-related data according to time, place, and person. (See analytic epidemiology)
Endemic — The habitual presence of a disease or infectious agent within a geographic
area or the prevalence of a given disease within such an area.
Epidemic — The occurrence of more cases of disease than expected in a given area or
among a specific group of people over a particular period of time. (See outbreak)
Epidemic Curve — A graph or histogram that plots the distribution of cases by time.
Epidemiology — The study of the distribution and determinants of health-related events
in specified populations, and the application of this study to control of health problems.
Experimental Study —A study in which the investigator specified the exposure category
for each individual (clinical trial) or community (community trial), and then follows the
individuals or community to detect the effects of the exposure.
Fomites — A subclass of vehicles, including inanimate objects such as article of
clothing, which may become contaminated and serve as a means of transmitting
agents.
Herd immunity — The resistance of a group to invasion and spread of an infectious
agent based on the immunity to a high proportion of individual members of the group.
Hypothesis — A supposition that can be tested and refuted.
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Epidemiology Terms
Passive immunity — Immunity attained either naturally (by maternal transfer) or
artificially (by inoculation of specific protective antibodies – convalescent or immune
serum, or immune serum globulin). Passive immunity is of brief duration, days or
months.
Active immunity — Immunity that lasts months to years is attained either naturally (by
infection), or artificially (by vaccination or immunization).
Incidence rate — The rate at which people without a disease develop the disease
during a specified time period (see prevalence, attack rate)
Incubation period — The time interval between the receipt of infection and the onset of
illness.
Index case — The first case to occur among a number of similar cases, which are
epidemiologically related. Index cases are often identified as a source of contamination
or source of infection for subsequent cases.
Latent period — The interval in time from exposure to chemical agents and the onset of
signs and symptoms of the illness.
Morbidity — The condition of being sick or diseased.
Mortality rate — The number of deaths from a given condition in a defined population in
a specified time period.
Observational Study — Study that does not involve any intervention. The investigator
observes the subjects and then records and analyzes the results.
Odds ratio — A measure of the strength of an association identified in a case control
study. The odds ratio is an approximation of the true relative risk
Outbreak — Synonymous with epidemic (more cases of disease than expected in a
given area or among a specific group of people over a particular period of time).
Sometimes the preferred word, since it may be less sensational.
Pandemic — A widespread epidemic disease affecting several countries or continents.
Pathogenicity — The capacity of an infectious agent to cause disease in a susceptible
host. See also Virulence.
Precision — Degree to which a measurement is sharply defined. (This can be
independent of validity).
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Epidemiology Terms
Prevalence — The amount of a given disease existing at a given point in time (or period
of time) in a community. (see incidence)
Relative risk — The ratio of the incidence rate of those exposed to a factor to the
incidence rate of those not exposed:
RR = incidence rate among those exposed
incidence rate among those not exposed
Relative risk is obtained directly from a cohort study and is a measure of the risk of
disease associated with a particular exposure.
Reservoir — The habitat in which an infectious agent normally lives, grows and
multiplies; reservoirs may be humans, animals, or environmental.
Secular trend — Refers to changes over a long period of time, years or decades, which
occur in both infectious and non-infectious conditions.
Surveillance (in Public Health) — The systematic collection, analysis, interpretation, and
dissemination of health data on an ongoing basis, to gain knowledge of disease
patterns to assist in disease prevention and control.
Transmission — The direct (contact or droplet spread) or indirect (vector-borne, vehicleborne, airborne) transfer of an infectious agent from a reservoir to a susceptible host.
Validity — Degree to which a measurement actually measures what it is supposed to
measure. (This can be independent of precision).
Vector — An arthropod, which transfers an infectious agent from an infected to a
susceptible host.
Virulence — The degree of pathogenicity of an infectious agent.
Vital Statistics — Data relating to birth, death, marriage, divorce, and illness (morbidity).
Zoonosis — An infection or an infectious disease transmissible under natural conditions
between animals and humans.
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