The incubation period is the time that elapses between exposure to an infectious agent and when symptoms first appear. It’s during this time that the pathogen is replicating in your system, and it takes time before enough cells are infected that the body takes notice and begins fighting it.
With some pathogens, it’s possible to be contagious during the incubation period, whereas other pathogens may not be contagious until after a person’s symptoms appear. With most coronaviruses, for example, people do not appear to be contagious until after symptoms appear, but with SARS-CoV-2, people can infect others before they begin experiencing symptoms.
The incubation period is usually a range, and it varies by pathogen, by the strength of the pathogen, and by the exposure dose. The incubation period can also vary by person: people with stronger or weaker immune systems may have longer or shorter incubation periods for the same pathogen.
Here are some examples of incubation periods for different pathogens that cause food-borne illness which illustrates how widely the incubation can vary by pathogen:
- Salmonella — Symptoms begin anywhere from six hours to six days.
- Botulism — From three to 30 days in infants but 18 to 36 hours in adults.
- Listeria — From one to four weeks, but up to 10 weeks.
- Norovirus — From 12 to 48 hours.
- Find other examples of incubation period lengths here.
The incubation period can also vary according to different variants of the same pathogen. For example, the original SARS-CoV-2 virus had an average incubation period of approximately 6.5 days, but the average incubation period was 4 days for the Delta variant and 3 to 4 days for the Omicron variant, according to a CDC report.