ANNEX 1 - Explanation of epidemiological terms1

¹ For an explanation of the term “epidemic”.

Carrier

An infected person or animal, which may be a source of infection but without showing any symptoms (incubatory carrier, convalescent carrier, transient carrier, chronic carrier).

Contagious (communicable) period

The period during which an infected person or animal, whether or not showing symptoms, can transmit a disease.

Contact

A person or animal that has been exposed to possible transmission of an infectious or parasitic agent by a patient, a sick animal or a carrier, or by contamination of the environment (including foods).

Contamination

The presence of an infectious or parasitic agent on body surfaces and inanimate supports, such as water, food, soil, air, dust and fomites.

Disease occurrence rates

Different rates may be utilized to quantify the occurrence of a disease in a population. They are calculated as fractions, in the form:

² This may be the entire population or a specific group (by age, sex, occupation, residence, etc.).

and expressed as rates per hundreds, thousands, or millions as convenient. The following rates should be used as appropriate:

(a) Clinical incidence (attack) rate

where the numerator is the number of overt cases (fitting the case definition) appearing during a specified period of time; the nature and extent of the population at risk should be defied.

(b) Infection rate

where the numerator is the number of overt (laboratory-proved) and silent infections (diagnosed only by laboratory tests) appearing during a specified period of time; the nature and extent of the population at risk should be defined.

(c) Case-fatality ratio

It should be stated whether the numerator is the number of etiologically confirmed deaths, or of presumptive deaths; the completeness of case detection (hospitalized cases, non-hospitalized cases, severe forms, mild forms, inapparent infections, all inclusive) should be specified.

(d) Prevalence rate

where the numerator may include only those clinically affected or, in addition, inapparent infections, at a given time (i.e., the date of the survey).

(e) Morbidity rate¹

¹ This term may be a source of confusion when indiscriminately used to refer to incidence or prevalence and should preferably be avoided.

where the numerator generally covers confirmed and suspected cases over a period of 1 year and the population concerned may be the entire population or a particular group.

(f) Mortality rate

where the deaths concerned are those directly or indirectly attributed to an agent, and the population may be the entire population or a particular group.

Endemic (enzootic for animals)

(Of a disease) continually present in a given area or community. Endemicity may be low or high, or even evidenced by only a few sporadic cases.

An endemic area is the limited zone in which a disease is known to occur constantly.

Epidemic (epizootic) focus

The limited area in which an outbreak has been occurring.

Fomites

Inanimate objects or materials, e.g., clothing, toilet articles, dressings, bedding, on which infectious agents may be carried.

Immunity

Resistance to a second infection by a particular agent (homotypic immunity) or by an antigenically related agent (heterotypic immunity).

Inapparent (subclinical) infection

Infection without clinical signs and symptoms in man or animal, detectable only by laboratory techniques, such as isolation of the agent, characterization of the antigen, and/or serological tests.

Infected area

The area in which, whether temporarily or permanently, an infectious or parasitic agent may be transmitted to man or animals.

Infection

The entry and multiplication or development in the body of an infectious (bacterial, chlamydial, fungal, rickettsial, viral) or parasitic agent (protozoal, helminthic).

Portals of entry include the respiratory tract, digestive tract, genital tract, skin (including injections) and conjunctivae.

Routes of excretion include, in addition to the above, excreta (stools, urine, vomitus), secretions (oral, nasal, sputum, saliva, tears, semen, vaginal secretions) and blood.

Outbreak

The occurrence of an epidemic disease at a particular time and place.

Pandemic

A worldwide epidemic.

Reservoir

Human beings, animals, plants, soil, water or any other substance in or upon which an infectious or parasitic agent is normally or occasionally found or multiplies in such a manner that it can be transmitted to a receptive person or animal.

Sporadic occurrence

The occurrence of a few cases of a disease without any relationship in time and with or without a relationship to a focus.

Transmission

The mechanism whereby an infectious or parasitic agent is transferred from an infected person or animal or a contaminated object to a receptive host (person or animal).

Transmission may be direct, by contact with the source of an infectious or parasitic agent, or indirect when it takes place through the intermediary, e.g., of an insect vector, air (when the agent is in suspension, e.g., in aerosols), dust, fomites, water or soil.

Vector

Arthropods of the class Insecta (mosquitos, flies, sandflies, fleas, lice, and other species), Arachnida (ticks and mites), or Crustacea (cyclops, crabs and crayfish), or vertebrate animals, e.g., rodents, that may transfer an infectious or parasitic agent from a reservoir to a receptive person or animal. The first two classes, namely Insecta and Arachnida, are commonly referred to as insects.

Zoonosis

An infectious or parasitic disease of animals, which may be transmitted to man. A zoonosis may be enzootic, epizootic or sporadic, and transmission may be direct or indirect.