Definitions including: incubation, communicability and latent period; susceptibility, immunity, and herd immunity

Definitions in communicable disease control

This section covers:

Definitions (incubation, communicability and latent period susceptibility, immunity, and herd immunity)

Note, where time periods are referred to, these are usually given as mean or median values with a minimum and maximum value as they are derived from observations of disease dynamics in multiple individuals within a population.

Incubation: Time interval between initial contact with an infectious agent and appearance of the first sign or symptom of disease in question.

Communicability: Period of communicability is the time during which an infectious agent may be transferred directly or indirectly from an infected person to another person, from an infected animal to humans, or from an infected person to animals. Also known as the ‘infectious period’.

Latent Period: The period between exposure and the onset of the period of communicability, which may be shorter or longer than incubation period.

Susceptibility: The state of being susceptible (easily affected / infected). A susceptible person does not possess sufficient resistance against a particular pathogen to prevent contracting that infection or disease when exposed to the pathogen.

Immunity: The condition of being immune, protected against an infectious disease conferred either by an immune response generated by immunisation or previous infection. Different types of immunity include:

Active Immunity: resistance developed by a host in response to a stimulus by an antigen (infecting agent or vaccine), usually characterised by antibody produced by the host.
Passive Immunity: Immunity conferred by an antibody produced in another host and acquired naturally by an infant from its mother or artificially by administration of antibody-containing preparations e.g. anti-serum or immunoglobulin.
Specific Immunity: A state of altered responsiveness to a specific substance acquired through immunisation or natural infection. In certain diseases this protection can last for the life of the individual.
Acquired Immunity: Resistance acquired by a host as a result of previous exposure to a natural pathogen or foreign substance for the host e.g. immunity to measles following measles infection.

Herd immunity: The level of immunity in a population which prevents epidemics, based on the resistance to infection of a proportion of individual members of the group sufficient to prevent widespread infection amongst non-immune members (Giesecke J Modern Infectious Disease Epidemiology). The proportion required varies according to agent, transmission characteristics and distribution of immune and susceptibles within the population.

Sources:

Last JM. A Dictionary of Epidemiology. New York: Oxford University Press, 1998.
Giesecke J. Modern Infectious Disease Epidemiology. London: Hodder Arnold, 2002