Food poisoning is any illness caused by foodborne bacteria, parasites, viruses, or chemicals. Every home, and every person, is host to a variety of bacteria that can cause serious illness if they get into food and multiply. You can get mild food poisoning without realizing it. When people come down with a "bug" accompanied by symptoms such as headache and stomach distress, it's often dismissed as "stomach flu" or "24-hour virus"—but it may be food poisoning.

And some types of microbes can cause severe illness that can be fatal in the elderly, in children, in people with certain disorders (such as diabetes), and in people whose immune systems are depressed, such as cancer patients. The Centers for Disease Control and Prevention (CDC) estimates that 76 million cases of food poisoning occur each year. Most go unreported since they are mild and last for only a day or two. But more than 300,000 cases are serious enough to require hospitalization, and an estimated 5,000 deaths each year are related to foodborne infections.

Symptoms of Food Poisoning

The onset of symptoms can occur anywhere from one hour to seven days after eating contaminated food, depending on the infectious agent.

  • Diarrhea (the most common symptom)
  • Abdominal pain and cramps
  • Prostration and shock (sometimes)
  • Nausea and vomiting (sometimes severe)
  • Fever (may not be present)
  • Itchy, numb or tingly skin
  • Headaches (may not be present)
  • Bloody stool (may not be present).
  • Muscle pain

Another sign of food poisoning is that people who have eaten the same food(s) all become ill.

What Causes Food Poisoning?

Food poisoning is primarily caused by a number of different bacteria and some viruses, with bacteria being responsible for the majority of cases. Just about every type of food—unless it has been sterilized—has bacteria in or on it, but most of them are harmless. In addition, bacteria can be introduced into foods from external sources. For example, fresh fruits and vegetables can become contaminated if they are washed or irrigated with water that contains animal manure or sewage. Contaminants can also be introduced during food processing by food handlers who are infected.

The mere presence of bacteria or viruses in food isn't enough to make you sick. They cause problems only when the food is improperly handled and prepared. Bacteria begin to multiply quickly in food left at room temperature and thrive on food that is kept warm on a stove. Moist foods, such as stuffing or cooked rice, are especially susceptible to bacterial growth. Refrigeration retards the growth of bacteria, and cooking at high temperature kills most of them. But if food has been left out long enough, some types of bacteria can form a toxin that will survive heat or freezing.

The bacterium Salmonella is by far the most frequent cause of foodborne illness, and it is rapidly becoming more prevalent. The Centers for Disease Control and Prevention (CDC) estimates that poisoning by Salmonella, called salmonellosis, accounts for several million cases annually—though only about 845,000 cases are actually reported. That's because many individuals and even doctors mistake salmonellosis for "intestinal flu." The increase in salmonellosis has been attributed principally to high-speed mechanical methods of slaughtering and eviscerating animals, especially poultry. According to a variety of estimates, at least half of all raw chicken marketed is contaminated by Salmonella and/or Campylobacter jejuni, another bacterium that causes diarrhea, fever, and abdominal cramps, and which may cause blood in the stool. Campylobacter has been implicated in millions of cases of food poisoning.

Any animal may harbor Salmonella. Raw eggs, too, can be a source. But in these cases poor handling may not be the only cause. Researchers suggest that the bacteria can come from inside the hen and are in the raw egg itself, rather than getting there by the usual route of cracked or dirty eggshells. While raw or soft-cooked eggs—or foods made with them, such as eggnog or Caesar salad—are potentially risky, commercial products made with eggs, such as mayonnaise, are safe because the eggs have been pasteurized.

Each year an estimated 25,000 cases of food poisoning are attributed to E. coli 0157:H7, a particularly dangerous strain of E. coli—a bacteria widely present in fecal matter—that can spread to humans when they consume food or water contaminated with microscopic amounts of cow feces. Most cases are associated with undercooked, contaminated ground beef, but cases have also occurred from many other sources, including people eating unpasteurized apple juice and apple cider made from fallen apples contaminated with manure.

Fish and shellfish are another common source of food poisoning, particularly when they are eaten raw. Any fish or shellfish, no matter how fresh, may carry some bacteria and viruses—and animals from sewage-polluted waters may carry large doses of them. Certain shellfish—clams, oysters, mussels—live by filtering water, and if the water they inhabit is polluted, they will retain bacteria and viruses along with the microscopic foodstuff they absorb. Raw shellfish can thus be a source of hepatitis, gastroenteritis, and other diseases. Raw fish, as used in sushi and sashimi, may be a source of parasites such as tapeworms and roundworms, as well as bacteria and viruses.


The Complete Home Wellness Handbook

John Edward Swartzberg, M.D., F.A.C.P., Sheldon Margen, M.D., and the editors of the UC Berkeley Wellness Letter

Updated by Remedy Health Media

Publication Review By: the Editorial Staff at

Published: 12 Sep 2011

Last Modified: 23 Dec 2014