Food poisoning occurs when you swallow food or water that contains bacteria, parasites, viruses, or toxins made by these germs. Most cases of food poisoning are from common bacteria such as Staphylococcus or E. coli.

Food poisoning can affect one person or a group of people who all ate the same contaminated food. It more commonly occurs after eating at picnics, school cafeterias, large social functions, or restaurants.

The germs may get into the food you eat (called contamination) in different ways:

• Meat or poultry can come into contact with the normal bacteria from the intestines of an animal that is being processed
• Water that is used during growing or shipping can contain manure or human waste
• Food handling or preparation in grocery stores, restaurants, or homes

Food poisoning often occurs from eating or drinking:

• Any food prepared by someone who does not use proper hand washing techniques
• Any food prepared using cooking utensils, cutting boards, and other tools that are not fully cleaned
• Dairy products or food containing mayonnaise (such as coleslaw or potato salad) that have sat out of the refrigerator too long
• Frozen or refrigerated foods that are not stored at the proper temperature or are not reheated properly
• Raw fish or oysters
• Raw fruits or vegetables that have not been washed well
• Raw vegetable or fruit juices and dairy (look for the word "pasteurized")
• Undercooked meats or eggs
• Water from a well or stream, or city or town water that has not been treated

Food poisoning can be caused by:

• Botulism (Clostridium botulinum)
• Campylobacter enteritis
• Cholera
• E. coli enteritis
• Fish poisoning
• Listeria
• Many different viruses
• Staphylococcus aureus
• Salmonella
• Shigella
• Yersinia

Infants and elderly people are at the greatest risk for food poisoning. You are also at higher risk if:

• You have a serious medical condition, such as kidney disease or diabetes
• You have a weakened immune system
• You travel outside of the United States to areas where there is more exposure to organisms that cause food poisoning

Pregnant and breastfeeding women have to be especially careful to avoid food poisoning.

The symptoms from the most common types of food poisoning generally start within 2 - 6 hours of eating the food. That time may be longer (even a number of days) or shorter, depending on the cause of the food poisoning.

Possible symptoms include:

• Abdominal cramps
• Diarrhea (may be bloody)
• Fever and chills
• Headache
• Nausea and vomiting
• Weakness (may be serious and lead to respiratory arrest, as in the case of botulism)

Your health care provider will examine you for signs of food poisoning, such as tenderness in the abdomen and dehydration. Your provider will also ask about foods you have eaten recently.

Tests to find the cause may be done on your:

• Blood
• Leftover food
• Stool
• Vomit

Even if you have food poisoning, however, these tests may not be able to prove it.

In rare but possibly serious cases, your health care provider may order one or more of the following procedures:

• A thin, tube-like tool placed in the anus to look for the source of bleeding or infection (sigmoidoscopy)
• A test to measure electric impulses in the muscles (electromyography) to check for botulism
• A test of fluid from the spine (lumbar puncture) if you have signs of a nervous system disorder

You will usually recover from the most common types of food poisoning within a couple of days. The goal is to make you feel better and avoid dehydration.

• Don't eat solid foods until the diarrhea has passed, and avoid dairy products, which can worsen diarrhea (due to a temporary state of lactose intolerance).
• Drink any fluid (except milk or caffeinated beverages) to replace fluids lost by diarrhea and vomiting.
• Give children an electrolyte solution sold in drugstores.

If you have diarrhea and are unable to drink fluids (for example, due to nausea or vomiting), you may need medical attention and fluids given through a vein (by IV). This is especially true for young children.

If you take diuretics, you need to manage diarrhea carefully. Talk to your health care provider -- you may need to stop taking the diuretic while you have the diarrhea. Never stop or change medications without talking to your health care provider and getting specific instructions.

For the most common causes of food poisoning, your doctor would NOT prescribe antibiotics.

You can buy medicines at the drugstore that help slow diarrhea. Do not use these medicines without talking to your health care provider if you have bloody diarrhea or a fever. Do not give these medicines to children.

If you have eaten toxins from mushrooms or shellfish, you will need medical attention right away. The emergency room doctor will take steps to empty out your stomach and remove the toxin.

Most people fully recover from the most common types of food poisoning within 12 - 48 hours. Serious complications can arise, however, from certain types of food poisoning.

Dehydration is the most common complication. This can occur from any of the causes of food poisoning.

Less common but much more serious complications include:

• Arthritis (Yersinia and Salmonella)
• Bleeding disorders (E. coli and others)
• Death (from mushrooms, certain fish poisonings, or botulism)
• Kidney problems (Shigella, E. coli)
• Nervous system disorders (Botulism, Campylobacter)
• Pericarditis (Salmonella)
• Respiratory distress, including the need for support on a breathing machine (botulism)

Call your health care provider if:

• Diarrhea lasts for more than 2 - 3 days
• There is blood in your stools
• You have diarrhea and are unable to drink fluids due to nausea or vomiting
• You have a fever over 101°F
• You have signs of dehydration (thirst, dizziness, light-headedness)

Go to the emergency room or call 911 if:

• Bleeding is excessive or your stools are maroon or black
• You may have poisoning from mushrooms, fish, or botulism
• Your heart is racing, pounding, or skipping

See: Preventing food poisoning

Sodha SV, Griffin PM, Hughes JM. Foodborne disease. In: Mandell GL, Bennett JE, Dolin R, eds. Principles and Practice of Infectious Diseases. 7th ed. Philadelphia, Pa: Elsevier Churchill Livingstone; 2009:chap 99.

Craig SA, Zich DK. Gastroenteritis. In: Marx JA, ed. Rosen's Emergency Medicine: Concepts and Clinical Practice. 7th ed. Philadelphia, Pa: Mosby Elsevier;2009:chap 92.