Time between infection and the onset of disease symptoms
In some diseases, as depicted in this diagram, the latency period is shorter than the incubation period. After the latency period (but before clinical infection) the infected person can transmit the disease without signs of any symptoms. Such infection is called subclinical infection.
Incubation period (also known as thelatent period orlatency period) is the time elapsed between exposure to apathogenic organism, a chemical, orradiation, and when symptoms and signs are first apparent.[1] In a typical infectious disease, the incubation period signifies the period taken by the multiplying organism to reach a threshold necessary to produce symptoms in the host.
Whilelatent orlatency period may be synonymous, a distinction is sometimes made whereby thelatent period is defined as the time from infection to infectiousness. Which period is shorter depends on the disease. A person may carry a disease, such asStreptococcus in the throat, without exhibiting any symptoms. Depending on the disease, the person may or may not becontagious during the incubation period.
During latency, an infection issubclinical. With respect toviral infections, in incubation the virus is replicating.[2] This is in contrast toviral latency, a form ofdormancy in which the virus does not replicate. An example of latency isHIV infection. HIV may at first have no symptoms and show no signs ofAIDS, despite HIV replicating in thelymphatic system and rapidly accumulating a largeviral load. People with HIV in this stage may beinfectious.
The terms "intrinsic incubation period" and "extrinsic incubation period" are used invector-borne diseases. The intrinsic incubation period is the time taken by an organism to complete its development in thedefinitive host. The extrinsic incubation period is the time taken by an organism to develop in theintermediate host.[citation needed]
For example, once ingested by a mosquito, malaria parasites must undergo development within the mosquito before they are infectious to humans. The time required for development in the mosquito ranges from 10 to 28 days, depending on the parasite species and the temperature. This is the extrinsic incubation period of that parasite. If a female mosquito does not survive longer than the extrinsic incubation period, then she will not be able to transmit any malaria parasites.[citation needed]
But if a mosquito successfully transfers the parasite to a human body via a bite, the parasite starts developing. The time between the injection of the parasite into the human and the development of the first symptoms of malaria is its intrinsic incubation period.[3]
Due to inter-individual variation, the incubation period is always expressed as a range. When possible, it is best to express the mean and the 10th and 90th percentiles, though this information is not always available.
For many conditions, incubation periods are longer in adults than they are in children or infants.
^Kahn, James O.; Walker, Bruce D. (1998). "Acute Human Immunodeficiency Virus Type 1 Infection".New England Journal of Medicine.339 (1):33–9.doi:10.1056/NEJM199807023390107.PMID9647878.
