Incubation 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, the 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 carry 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. 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. These persons 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.

Determining factors

The specific incubation period for a disease process is the result of multiple factors, including:
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.
Diseasebetweenandperiod
Cellulitis caused by Pasteurella multocida01days
Chicken pox921days
Cholera0.54.5days
Common cold13days
COVID-19211.5/12.5/14days
Dengue fever314days
Ebola121, 42 days
Erythema infectiosum 1318days
Giardia321days
HIV23weeks to months, or longer
Infectious mononucleosis 2842days
Influenza13days
Kuru disease10.313.2years
Marburg510days
Measles912days
MERS214days
Mumps1418days
Norovirus12days
Pertussis 714days
Polio714days
Rabies13months, but may vary from <1 week to >1 year.
Rocky Mountain spotted fever214days
Roseola515days
Rubella 1421days
Salmonella1224hours
Scarlet fever14days
SARS110days
Smallpox717days
Tetanus721days
Tuberculosis212weeks
Typhoid721days