Incubation period

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 the latent period or latency period) is the time elapsed between exposure to a pathogenic organism, a chemical, or radiation, 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.

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. With respect to viral infections, in incubation the virus is replicating.^[2] 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.^{[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]

Determining factors

The specific incubation period for a disease process is the result of multiple factors, including:^{[citation needed]}

• Dose or inoculum of an infectious agent
• Route of inoculation
• Rate of replication of infectious agent
• Host susceptibility
• Immune response

Examples for diseases in humans

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.

Disease	between	and
Cellulitis caused by Pasteurella multocida	0 days[4]	1 day
Chicken pox	9 days[5]	21 days
Cholera	0.5 days[6]	4.5 days
Common cold	1 day[7][8]	3 days
COVID-19	2 days [9]	11.5[10]/12.5[11]/14 days
Dengue fever	3 days[12]	14 days
Ebola	1 day[13]	21 (95%), 42 (98%) days
Erythema infectiosum (Fifth disease)	13 days[14]	18 days
Giardia	3 days	21 days
HIV	2 weeks to months, or longer[15]	3 weeks to months, or longer
Infectious mononucleosis (glandular fever)	28 days[16]	42 days
Influenza	1 day[17]	3 days
Kuru disease	10.3 years (mean)[18]	13.2 years
Leprosy	1 year[19]	20 or more years
Marburg	5 days[20]	10 days
Measles	9 days[21]	12 days
MERS	2 days[22]	14 days
Mumps	14 days[23]	18 days
Norovirus	1 day[24]	2 days
Pertussis (whooping cough)	7 days[25]	14 days
Polio	7 days[26]	14 days
Rabies	1 months, but may vary from <1 week to rarely >1 year.[27][28]	3 months
Rocky Mountain spotted fever	2 days[29]	14 days
Roseola	5 days[30]	15 days
Rubella (German measles)	14 days[31]	21 days
Salmonella	12 days[31]	24 days
Scarlet fever	1 day[32]	4 days
SARS	1 day[33]	10 days
Smallpox	7 days[34]	17 days
Tetanus	7 days[35]	21 days
Tuberculosis	2 weeks[36]	12 weeks
Typhoid	7 days	21 days

See also

• Latent period
• Infectious period
• Gestation period
• Prodrome
• Quarantine
• Window period, the time between infection and when lab tests can identify the infection.

References

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