What is the incubation period?

Definition

The
incubation period is the time between exposure to a virus or bacterium and the development of symptoms.

Length of Incubation Periods

Symptom onset is determined by factors such as pathogen
growth and invasion, excretion of toxins, and initiation of host-defense
mechanisms. The length of incubation time varies greatly and depends on pathogen
replication rate, mechanism of disease development, infection route, and other
underlying factors. The incubation period of infectious diseases may be as short
as a few hours (common in food poisoning) or may last many months
(such as with rabies). Incubation periods can also be several years, such
as those for leprosy, tuberculosis, and acquired immunodeficiency syndrome, and as
long as thirty years, as with variant Creutzfeldt-Jakob disease. Incubation
periods for other common diseases are generally shorter and include one to two
days for influenza, two to five days for the common cold, two to fourteen days for
Rocky Mountain spotted fever, twelve days for smallpox, eight to twelve days for
measles, and fourteen to twenty-one days for German measles (rubella).

In infections in which the pathogen travels a short distance before it reaches
the target organ, the incubation period is brief (commonly one to three days).
Incubation periods of more generalized infections are usually longer because the
pathogen needs to move through the body before reaching target organs. Mechanisms
involved with long incubation periods, those from months to years, of persistent
infections are not well understood. Disease may also result from delayed immune
reactions to viral antigen, such as with adenoviruses;
from unknown mechanisms during which no immune response can be detected, as in the
scrapie-kuru group; or mutations in host genetic material that leads to cell
transformation and ultimately, cancer.

Carriers

Depending on the disease, a person may or may not be contagious during the
incubation period. A person may be a carrier of a disease, such as
Streptococcus infection, without exhibiting symptoms.

Epidemiology

In the mid-sixteenth century, an Italian physician, Girolamo
Fracastoro, provided the first documentation of the
incubation period for rabies. The earliest study of the incubation period of
pandemic influenza was conducted in 1919. The study, which recorded daily
incidence on ships departing from Australian ports, provided estimates of the
incubation period of Spanish flu.

Impact

The incubation period for infectious disease is directly relevant to prevention
and control. Incubation periods provide valuable insight into clinical and public
health practices and are important for epidemiologic and ecological studies. In
clinical practice, the incubation period can be used to determine causes and
sources of infection in individual cases and in developing treatment strategies to
extend the incubation period, such as during antiretroviral therapy for
human
immunodeficiency virus infection, and to project disease
prognosis. During an outbreak of emerging diseases, the incubation period can
assist in estimating quarantine periods for potentially exposed persons.

Bibliography

Aschengrau, Ann, and George R. Seage. Essentials of Epidemiology in Public Health. 2d ed. Sudbury, Mass.: Jones and Bartlett, 2007.

Giesecke, Johan. Modern Infectious Disease Epidemiology. 2d ed. London: Hodder Arnold, 2001.

Hawker, Jeremy, et al. Communicable Disease Control Handbook. Malden, Mass.: Blackwell, 2005.

Murphy, Kenneth, Paul Travers, and Mark Walport. Janeway’s Immunobiology. 7th ed. New York: Garland Science, 2008.