The incubation period of a disease is the period of time between initial exposure to the disease causing agent and the outbreak of symptoms. Although it applies to any type of disease, it is most commonly used when speaking of infectious diseases. The patient may still test positive for the disease, and may still be contagious. They often vary greatly from patient to patient and often have to be stated to encompass the range of 80% of patients, leaving out the 10% who develop it the quickest and the 10% that develop it the slowest.
The incubation periods of different diseases vary widely. For example, cholera usually shows symptoms sometime between 1 and 3 days of exposure. Mononucleosis often doesn't show symptoms for at least 28 days. AIDS may not show any symptoms for years. Viruses usually manifest the quickest due to their ability to reproduce quickly (although there are obvious exceptions). Bacteria are usually next. Parasites are often the slowest. Prion diseases are usually the slowest of all.
An incubation period can be helpful in a differential diagnosis. If a person has a history of travel from an area with an outbreak, an endemic disease can often be ruled out if the patient shows symptoms well after the incubatin period has passed.