Food poisoning is a brief illness that is caused by toxins produced by bacteria. The toxins cause abdominal pain (cramps) and vomiting, and also cause the small intestine to secrete large amounts of water that leads to diarrhea. The symptoms of food poisoning usually last less than 24 hours. With some bacteria, the toxins are produced in the food before it is eaten, while with other bacteria, the toxins are produced in the intestine after the food is eaten.
Symptoms usually appear within several hours when food poisoning is caused by toxins that are formed in the food before it is eaten. It takes longer for symptoms to develop when the toxins are formed in the intestine (because it takes time for the bacteria to produce the toxins). Therefore, in the latter case, symptoms usually appear after 7-15 hours.
Staphylococcus aureus is an example of a bacterium that produces toxins in food before it is eaten. Typically, food contaminated with Staphylococcus (such as salad, meat or sandwiches with mayonnaise) is left un-refrigerated at room temperature overnight. The Staphylococcal bacteria multiply in the food and produce toxins. Clostridium perfringens is an example of a bacterium that multiplies in food (usually canned food), and produces toxins in the small intestine after the contaminated food is eaten.