Incubation period

While latent or latency period may be synonymous, a distinction is sometimes made whereby the latent 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 as Streptococcus 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 (also called asymptomatic). With respect to viral infections, in incubation 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. People with HIV in this stage may be infectious.

Intrinsic and extrinsic incubation period

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 develop in the intermediate host.

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.

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.