HM 101 Lecture Notes - Lecture 7: Asymptomatic Carrier, Neonatal Tetanus, Chemical Substance
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Module #7 List of Epidemiologic Definitions
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.
Carrier: A person or animal without apparent disease who harbors 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 inapparent throughout its course (known as asymptomatic carrier), or
during the incubation period, convalescence, and postconvalescence of an individual with a
clinically recognizable disease. The carrier state may be of short or long duration (transient
carrier or chronic carrier).
Elimination of a disease: Reduction to zero of the incidence of a specified disease in a defined
geographical area as a result of deliberate efforts; continued intervention measures are
required. Example: neonatal tetanus. (from
Eradication of a disease: Permanent reduction to zero of the worldwide incidence of infection
caused by a specific agent as a result of deliberate efforts; intervention measures are no longer
needed. Example: smallpox. (from
Reservoir: The habitat in which an infectious agent normally lives, grows and multiplies;
reservoirs include human reservoirs, animals reservoirs, and environmental reservoirs.
Vector: An animate intermediary in the indirect transmission of an agent that carries the agent
from a reservoir to a susceptible host.
Host: A person or other living organism that can be infected by an infectious agent under
Zoonoses: An infectious disease that is transmissible under normal conditions from animals to
All definitions except elimination and eradication from Epidemiology Glossary at