Lesson 1: Introduction to Disease Transmission and Epidemiology

1-3

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1-3. DEFINITIONS

In order to approach the subject of disease transmission and epidemiology in a clear and logical manner, it is necessary to establish a common ground in the use of terminology. The following terms will be used frequently throughout this subcourse.

a. Disease. Disease is an impairment of the normal state of the living animal or plant body that affects the performance of the vital functions. The presence of disease usually results in visible signs or symptoms.

b. Communicable Disease. A communicable disease is an illness that can be transmitted person to person or from animal to person.

c. Agent. An agent is a disease-producing organism or substance.

d. Infection. Infection is the entrance and multiplication of infectious (disease-producing) agents into the body of man or animal.

e. Reservoir. A reservoir is the source of a disease, harboring the infectious agent(s). The agent either multiplies or undergoes some development with the organism or substance acting as the reservoir.

f. Mode of Transmission. The mode of transmission is the means by which a disease is transmitted from one person or animal to another.

g. Vector. A vector is an animal or arthropod that plays a part in the transmission of disease. A disease vector may be either the reservoir or the vehicle in disease transmission.

h. Host. The host is the living body upon which a parasite or infectious agent lives--the final recipient of a disease agent. The host of a disease may be either a case or a carrier.

i. Case. A case refers to a person who is actually ill with a disease.

j. Carrier. A carrier is an individual (or animal) who is infected with a disease, agent and is capable of transmitting the disease, but who usually does not exhibit clinical symptoms.

k. Incubation Period. The incubation period is the time interval between the entrance of an infectious agent into a host and the appearance of symptoms.

l. Spectrum of Infection. The spectrum of infection is the broad gradation of disease infection from no apparent symptoms (such as the carrier state) through severe illness and death.

m. Endemic. Endemic refers to the usual level of occurrence of a disease within a given geographical area.

n. Epidemic. Epidemic is the occurrence of a disease clearly in excess of the normal expectancy within a given geographical area.

o. Pandemic. Pandemic is the occurrence of disease over a wide geographical area and affecting an exceptionally high percentage of the population.

p. Epidemiology. Epidemiology is the study of the determinants and distribution of disease and injury in a given population.

q. Vehicle. A vehicle is an inanimate object that facilitates the transmission of a disease-causing agent.

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