An incubation period is the time elapsed between exposure to a pathogenic organism, a chemical, or radiation, and when symptoms and signs are first apparent. It is the time it takes for an infection to develop after a person has been exposed to a disease-causing organism such as bacteria, viruses, or fungi. The incubation period ends when the first signs or symptoms of the disease appear. 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.

The incubation period is different for every condition and varies depending on the pathogen and the individuals immune system. 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.

Health care professionals and government officials use the incubation period to decide how long people need to stay away from others during an outbreak. It is also important to note that viruses are constantly changing, which sometimes leads to new strains called “variants.” Different COVID-19 variants can have different incubation periods.