The time between eating contaminated food and the first appearance of illness symptoms, which varies from hours to weeks depending on the organism.
Many bacteria need time to multiply in the body to numbers that cause illness. Organisms like Salmonella and Campylobacter usually take one to three days, and Listeria can take weeks, so the last meal eaten is not always the cause.
Pre-formed toxins act fastest. Staphylococcus aureus and Bacillus cereus toxins can cause vomiting within one to six hours of eating the affected food.
Listeria monocytogenes is notable for a long and variable incubation, ranging from a few days up to around 70 days, which makes listeriosis particularly difficult to trace.
They work backwards from when people fell ill, using the known incubation range of suspected organisms, to identify which meal or food is the likely source of an outbreak.
Not necessarily. Because incubation periods vary widely by organism, the food eaten just before symptoms is not always the culprit. Timing is one clue among several used in an investigation.