Food Poisoning

Sudden illness caused by consuming food or drink contaminated by a toxin or infectious organism

  • Poor food hygiene is a risk factor
  • Age, gender, and genetics are not significant factors

Food poisoning is the term used to describe a sudden illness that is caused by consuming food or drink that may taste normal but is contaminated with a toxin or infectious organism.

The diagnosis of food poisoning is easily made if a group of people all develop the same symptoms, usually vomiting and diarrhoea, after they have consumed the same food or drink. The symptoms may start hours or days after consuming the food or drink.

Usually, the symptoms are confined to the gastrointestinal tract. However, some food poisoning may cause more widespread symptoms. For example, the Clostridium botulinum bacterium causes muscle weakness and paralysis (see Botulism), and listeriosis may cause flu-like symptoms and lead to meningitis.

Food poisoning is very common, with an estimated 850,000 cases each year in the UK. However, it can usually be avoided by careful preparation, storage, and cooking of food (see Food hygiene).

What are the causes?

Most cases of food poisoning result from contamination of food or water by bacteria, viruses, or, less commonly, protozoal parasites. Poor food hygiene can enable microorganisms to multiply. In some cases of bacterial food poisoning, it is not the presence of the bacteria themselves that cause poisoning but the effect of toxins produced by the bacteria.

If infectious organisms are ingested with the food, they can multiply in the digestive tract. If the food poisoning is caused by bacterial toxins, they may be produced in the food before it is eaten.

Most types of food poisoning cause diarrhoea and/or vomiting, often with abdominal pain. The severity of symptoms, the speed at which they develop, and the duration of the illness depend on the cause of food poisoning.

Staphylococci bacteria

A number of foods, such as poultry, eggs, pâté, and previously prepared sandwiches can be infected by staphylococci bacteria. These bacteria produce toxins that are ingested with food. The toxins usually produce diarrhoea and/or vomiting within 4 hours. In most cases, symptoms clear up within 24 hours.

Escherichia coli bacteria

Certain types of E. coli can contaminate meat and water and produce toxins of varying potency. Types of E. coli are usually responsible for causing traveller’s diarrhoea, which is usually mild. However, some types of E. coli (such as the strain called E. coli 0157) may cause a severe illness because they produce a potent type of toxin that can damage blood cells and lead to kidney failure.

Salmonella bacteria

Up to half a million people develop the disease salmonellosis every year in the UK from eating eggs or poultry that are infected with salmonella bacteria. It typically causes symptoms such as vomiting, mild fever, and severe diarrhoea that may be bloodstained. The symptoms usually begin 12–72 hours after eating the contaminated food and last for 1–3 days.

Campylobacter bacteria

These bacteria are one of the most common causes of diarrhoea. They may contaminate meat and more rarely water or unpasteurized milk. Symptoms usually develop about 2–5 days after eating contaminated food and may include severe, watery diarrhoea. The diarrhoea may contain blood and/or mucus. In most cases, the symptoms subside within 2–3 days, but bacteria may be present in faeces for up to 5 weeks after infection.

Other infections

Viral infections can be contracted from contaminated food or water. Shellfish are a common source of infection, especially with noroviruses. Symptoms often start suddenly after contaminated food has been eaten, but recovery is usually rapid. Protozoal infections that may be contracted from contaminated food or water include cryptosporidiosis, amoebiasis, and giardiasis. Crypto-sporidiosis may cause symptoms such as vomiting and loose, watery diarrhoea to develop about a week after contaminated substances have been consumed. Symptoms of amoebiasis may include watery, often bloody, diarrhoea persisting for several days or weeks. In giardiasis, there may be diarrhoea, bloating, and flatulence that often last more than a week.

Noninfectious causes

In some cases, food poisoning may be caused by poisonous mushrooms or contamination of fruit or vegetables with high concentrations of pesticides. Symptoms may include vomiting and diarrhoea.


This magnified image shows particles of norovirus, which often contaminates shellfish and causes food poisoning.

What might be done?

Usually, symptoms disappear without treatment. If your symptoms are mild, use self-help measures to prevent dehydration (see Preventing dehydration). If the symptoms are severe or last more than 3–4 days, consult your doctor. If an elderly person or child is affected, you should consult a doctor immediately. Keep a sample of any remaining food in addition to the faeces, which can be tested for the presence of infectious microorganisms. If the cause is noninfectious, such as poisonous mushrooms, you may need to be treated urgently to eliminate the poison from the body.

Treatment of food poisoning is usually aimed at preventing dehydration. In severe cases, fluids and salts may be administered intravenously in hospital. Antibiotics are given only if specific bacteria have been identified. People usually recover rapidly from a bout of food poisoning and rarely experience long-lasting consequences.

In rare cases, there is a risk of septicaemia if bacteria spread into the bloodstream. Both dehydration and septicaemia can lead to shock, a condition that may be fatal. Infection with E. coli 0157 may also cause serious illness, which may occasionally be fatal.

From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.

The subjects, conditions and treatments covered in this encyclopaedia are for information only and may not be covered by your insurance product should you make a claim.

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