Latency period

Incubation period is the time elapsed between exposure to a pathogenic organism, a chemical, or radiation, and when symptoms and signs are first apparent. In a typical infectious disease, incubation period signifies the period taken by the multiplying organism to reach a threshold necessary to produce symptoms in the host.

While latent or latency period may be synonymous, a distinction is sometimes made between incubation period, the period between infection and onset of the disease, and latent period, the time from infection to infectiousness. Which is shorter depends on the disease. A person may be a carrier of a disease, such as in the throat, without exhibiting any symptoms. Depending on the disease, the person may or may not be contagious during the incubation period.

During latency, an infection is subclinical. With respect to viral infections, in latency the virus is replicating. This is in contrast to viral latency, a form of dormancy in which the virus does not replicate. An example of latency is HIV infection. HIV may at first have no symptoms and show no signs of AIDS, despite HIV replicating in the lymphatic system and rapidly accumulating a large viral load. These persons may be infectious.

The terms "intrinsic incubation period" and "extrinsic incubation period" are used in vector-borne diseases. The intrinsic incubation period is the time taken by an organism to complete its development in the definitive host. The extrinsic incubation period is the time taken by an organism to complete its development in the intermediate host.

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