Infectious disease terminology

This section is intended as a refresher and glossary for terminology you may encounter in the following case modules.

Active surveillance

Involves the systematic and regular recording of cases of a defined population and/or time. It requires a planned study design to actively detect cases of disease by sampling of clinically normal animals (picks up subclinical and carrier animals). Active surveillance assesses disease prevalence to produce a quantifiable estimate of disease and disease status.


An infected animal that harbours a specific infectious agent in the absence of discernible clinical disease and serves as a potential source of infection for other animals. The carrier state may be inapparent throughout infection (healthy or asymptomatic carrier), may occur only during the incubation period, or may occur after the animal has recovered from clinical disease (a convalescent carrier). Carriers can be infected without being infectious (for example Brucella abortus in a steer).

Emerging Infectious Diseases (EIDs)

Emerging Infectious Diseases (EIDs) are infections that have recently appeared within a population or those whose incidence or geographic range is rapidly increasing or threatens to increase in the near future. Emerging infections can be caused by:

  • Previously undetected or unknown infectious agents
  • Known agents that have spread to new geographic locations or new populations
  • Previously known agents whose role in specific diseases has previously gone unrecognized.
  • Re-emergence of agents whose incidence of disease had significantly declined in the past, but whose incidence of disease has reappeared. This class of diseases is known as re-emerging infectious diseases.


An inanimate object capable of transmitting an infectious agent. These include vehicles, equipment and clothing.


An animal capable of being infected with an infectious agent. Replication or development of the infectious agent often occurs in a host.

Incubation period

The interval between effective exposure to an infectious agent and the appearance of the clinical signs.

Infectious period

The interval during which the infectious organism is shed by the infected host.


The ability of an agent to establish an infection. Unrelated to the potential severity of disease produced.

Latent period or prepatent period

The interval between infection to shedding of the infectious organism.

Passive surveillance

Relies on notifications of disease suspicions and cases by veterinarians, farmers, abattoirs and laboratories. It involves continuous monitoring of existing disease status of the population using data from laboratory reports, routine meat inspection findings and statutory notification of disease.

Passive surveillance generally samples clinically affected cases of specified diseases. The effectiveness of passive surveillance depends on: the disease being notifiable, veterinary and farmer awareness, willingness to report, presence of a compensation scheme and diagnostic capacity.

Passive surveillance is important for early warning of new or increased disease occurrence and as a first stage in identifying new and emerging disease. Private veterinarians are important for the passive surveillance of EADs.


The ability of an agent to produce disease in an infected host. Pathogenicity is also affected by the environment.


The living organisms or inanimate matter (e.g. soil) in which an infectious agent normally lives and multiplies (where it maintains and perpetuates itself) and from which it can be transmitted. Often the reservoir is not directly affected by disease.


The collection and interpretation of data with the purpose of detecting changes in the population’s health. Implies that some form of directed action will be taken if the data indicate disease prevalence or incidence is above a certain threshold.

Transboundary Animal Diseases (TADs)

Transboundary Animal Diseases (TADs) are defined by the FAO as diseases of significant economic, trade and/or food security importance for a considerable number of countries; which can easily spread to other countries and reach epidemic proportions; and where control/management, including exclusion, requires cooperation between several countries. TADs includes emerging infectious diseases.


Any living carrier that transmits infectious agents. Mechanical vectors transmit the agent by physical contact without being involved in the organism’s development (e.g. house flies). In the case of biological vectors, the infectious agent develops or replicates within them before the agent is transmitted to a susceptible animal (e.g. mosquitoes).