All foods naturally contain small amounts of bacteria. But poor handling of food, improper cooking or inadequate storage can result in bacteria multiplying in large enough numbers to cause illness.
Parasites, viruses, toxins and chemicals also can contaminate food. Food-borne illness from these sources, however, is less common than food-borne illness caused by bacteria.
Signs and symptoms of food poisoning vary with the source of contamination. Generally diarrhea, nausea, abdominal pain and, sometimes, vomiting occur within hours after eating contaminated food.
Whether you become ill after eating contaminated food depends on the organism, the amount of exposure, your age and your health.

Food-borne illness often improves on its own within 48 hours. Call your doctor if you feel ill for longer than two or three days or if blood appears in your stools.
Dial 911 or call for emergency medical assistance if:
— You have severe symptoms, such as watery diarrhea that turns very bloody within 24 hours.
— You belong to a high-risk group.
— You suspect botulism poisoning.
Botulism is a potentially fatal food poisoning that results from the ingestion of a toxin formed by certain spores in food. Botulism toxin is most often found in home-canned foods, especially green beans and tomatoes. Signs and symptoms usually begin 12 to 36 hours after eating the contaminated food and may include headache, blurred vision, muscle weakness and eventual paralysis. Some people also have nausea and vomiting, constipation, urinary retention, difficulty breathing and dry mouth. These signs and symptoms require immediate medical attention.