All foods naturally contain small amounts of bacteria. But improper handling, cooking or storage of food can result in bacteria multiplying in large enough numbers to cause illness. Parasites, viruses, toxins and chemicals also can contaminate food and cause illness.

Signs and symptoms of food poisoning vary with the source of contamination, and whether you are dehydrated or have low blood pressure. Generally they include:

• Diarrhea, which may be bloody
• Nausea
• Abdominal pain
• Vomiting
• Dehydration
• Low-grade fever (sometimes)

With significant dehydration, you might notice:

• Feeling lightheaded or faint, especially on standing
• Fatigue
• Dark-colored urine
• Less frequent urination
• Excessive thirst

Whether you become ill after eating contaminated food depends on the organism, the amount of exposure, your age and your health. High-risk groups include:

• Older adults. As you get older, your immune system may not respond as quickly and as effectively to infectious organisms as it once did.
• Infants and young children. Their immune systems haven't fully developed.
• People with chronic diseases. Having a chronic condition, such as diabetes or AIDS, or receiving chemotherapy or radiation therapy for cancer, reduces your immune response.
• Pregnant women. Pregnancy alters your immune system, making it harder to fight off infections that may affect you and your developing fetus.

If you develop food poisoning:

• Sip liquids, such as a sports drink or water, to prevent dehydration. Drinking fluids too quickly can worsen the nausea and vomiting, so try to take small frequent sips over a couple of hours, instead of drinking a large amount at once.
• Take note of urination. You should be urinating at regular intervals, and your urine should be light and clear. Infrequent passage of dark urine is a sign of dehydration. Dizziness and lightheadedness also are signs of dehydration. If any of these signs and symptoms occur and you can't drink enough fluids, seek medical attention.
• Avoid anti-diarrheal medications. They may slow elimination of organisms or toxins from your system. If in doubt, check with your doctor about your particular situation.

Infants or young children should not be given anti-diarrheal medications because of potentially serious side effects.

Foodborne illness often improves on its own within a few days.

Call your doctor if:

• Vomiting persists for more than two days
• Diarrhea persists for more than several days
• Diarrhea turns bloody, black or tarry
• Fever is 101 F (38.3 C) or higher
• Lightheadedness or fainting occurs with standing
• Confusion develops
• Worrisome abdominal pain develops

Seek emergency medical assistance if:

• You have severe symptoms, such as severe abdominal pain or 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 or tomatoes. Signs and symptoms of botulism usually begin 12 to 36 hours after eating the contaminated food and may include a headache, blurred vision, muscle weakness and eventual paralysis. Some people also have nausea and vomiting, constipation, urinary retention, difficulty breathing, and a dry mouth. These signs and symptoms require immediate medical attention.