Most people I talk to have a misconception that it is always the last thing they ate that made them sick. More often than not, this is not the case. You cannot see, smell or taste most harmful microorganisms, unless you are a gifted superhero, even then I wouldn't bet the farm.
Onset time is the period between where an individual first consumes a contaminated food, to when symptoms of disease start to show up. An example of a viral infection like Hepatitis A has a long onset time, around seven weeks. Meaning, if you enjoyed a raw or lightly steamed clam that was from a contaminated shellfish bed, you may not start feeling the effects until almost two months after the fact. A bacterial intoxication like Staphylococcus aureus has a short onset time of one to six hours. Even shorter, if you have eaten some undercooked tuna that has a naturally occurring toxin like Scombrotoxin, the onset time is just a few minutes. In this case, you may not even have enough time to make it to the restroom.
Different factors involving the victim can also cause a variation in onset time, such as: health status, age, weight and the amount of pathogen that was consumed. A lot of people have contracted a foodborne illness and not even realize it.They chalk it up as a twenty-four hour flu. Depending on what you ate, the symptoms and onset time you can only make an educated guess at what made you sick. That it why it is so difficult to trace the source of a foodborne illness outbreak. Most people, like myself, don't remember what they consumed four days ago, let alone every ingredient involved.