HLTB21H3 Chapter Notes -Frequency Distribution, Antigenicity, Intrinsic Factor
HLTA01H3S Plagues and Peoples
Glossary of Epidemiologic Terms
Populations at risk: These can be identified by studying the distribution of the disease
within host populations by species, breed, age and sex. Descriptions of population
densities and movements are also of great value, particularly when the disease is
transmitted by contact.
Distribution of disease: events in time and space. This generally involves looking for the
"clustering" of disease events in time, space or both.
The clustering of disease: events in time may indicate that the host population was
exposed to a common source of the disease or its determinant. Seasonal clustering of
disease events often indicates the influence of climatic determinants in some form or
The distribution of disease: events in populations in time and space can be described by
three basic descriptive terms. These are: endemic, epidemic and sporadic.
An endemic disease: is a disease that occurs in a population with predictable regularity
and with only minor deviations from its expected frequency of occurrence. In endemic
diseases, disease events are clustered in space but not in time. [localized]
An epidemic disease: is a disease that occurs in a population in excess of its normally
expected frequency of occurrence. In an epidemic disease, disease events are clustered in
time and space. Note that a disease may be epidemic even at a low frequency of
occurrence, provided that it occurs in excess of its expected frequency. [takes on a
continent or a country]
A pandemic: is a large epidemic affecting several countries or even one or more
continents. [the disease spans the entire world]
A sporadic disease: is a disease that is normally absent from a population but which can
occur in that population, although rarely and without predictable regularity.
Age-adjusted mortality rate: A mortality rate statistically modified to eliminate the
effect of different age distributions in the different populations.
Agent: A factor, such as a microorganism, chemical substance, or form of radiation,
whose presence, excessive presence, or (in deficiency diseases) relative absence is
essential for the occurrence of a disease. [cause of the disease]
Age-specific mortality rate: A mortality rate limited to a particular age group. The
numerator is the number of deaths in that age group; the denominator is the number of
persons in that age group in the population.
Antigenic variation: Some species of disease agent seek to evade the hosts' defence
mechanisms by altering their antigenic characteristics.
Antigenic drift: which involves only minor changes in antigenicity, so that hosts
previously infected with the agent retain a certain degree of immunity to the drifted
Antigenic shift: which involves a major change in antigenicity, so that previously
infected individuals possess little or no immunity to the shifted agent.
Antigenic shifts: are of particular significance when the control of a disease is being
attempted by vaccination, since in effect they represent the introduction of a new agent
against which the existing vaccine is likely to confer little or no immunity (example
Bias: Deviation of results or inferences from the truth, or processes leading to such
systematic deviation. Any trend in the collection, analysis, interpretation, publication, or
review of data must be free from any bias.
Biologic transmission: The indirect vector-borne transmission of an infectious agent in
which the agent undergoes biologic changes within the vector before being transmitted to
a new host.
Carrier: A person or animal without apparent disease who harbours a specific infectious
agent and is capable of transmitting the agent to others. The carrier state may occur in an
individual with an infection that is in-apparent throughout its course (known as
asymptomatic carrier), or during the incubation period, convalescence, and post-
convalescence of an individual with a clinically recognizable disease. The carrier state
may be of short or long duration (transient carrier or chronic carrier).
Case: In epidemiology, a countable instance in the population or study group of a
particular disease, health disorder, or condition under investigation. Sometimes it could
be an individual with the particular disease.
Cause of disease: A factor (characteristic, behaviour, event, etc.) that directly influences
the occurrence of disease. A reduction of the factor in the population should lead to a
reduction in the occurrence of disease.
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.