Census: The enumeration of an entire population, usually with details being recorded on
residence, age, sex, occupation, ethnic group, marital status, birth history, and
relationship to head of household.
Chain of infection: A process that begins when an agent leaves its reservoir or host
through a portal of exit, and is conveyed by some mode of transmission, then enters
through an appropriate portal of entry to infect a susceptible host.
Class interval: A span of values of a continuous variable which are grouped into a single
category for a frequency distribution of that variable.
Cluster: An aggregation of cases of a disease or other health-related condition,
particularly cancer and birth defects, which are closely grouped in time and place. The
number of cases may or may not exceed the expected number; frequently the expected
number is not known.
Cohort: A well-defined group of people who have had a common experience or
exposure, who are then followed up for the incidence of new diseases or events, as in a
cohort or prospective study. A group of people born during a particular period or year is
called a birth cohort.
Cohort study: A type of observational analytic study. Enrolment into the study is based
on exposure characteristics or membership in a group. Disease, death, or other health-
related outcomes are then ascertained and compared.
Common source outbreak: An outbreak that results from a group of persons being
exposed to a common noxious influence, such as an infectious agent or toxin. If the group
is exposed over a relatively brief period of time, so that all cases occur within one
incubation period, then the common source outbreak is further classified as a point source
outbreak. In some common source outbreaks, persons may be exposed over a period of
days, weeks, or longer, with the exposure being either intermittent or continuous.
Contact: Exposure to a source of an infection, or a person so exposed.
Contagious: Capable of being transmitted from one person to another by contact or close
Control: In a case-control study, comparison group of persons without disease.
Demographic information: The ``person'' characteristics--age, sex, race, and
occupation--of descriptive epidemiology used to characterize the populations at risk.