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Food poisoning - Causes of food poisoning

NHS Choices Medical Reference

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Food can become contaminated at any stage during its production, processing or cooking.

For example, you can get food poisoning by:

  • not cooking food thoroughly (particularly poultry, pork, burgers, sausages and kebabs)
  • not storing food that needs to be chilled at below 5°C correctly
  • keeping cooked food unrefrigerated for more than an hour   
  • eating food that has been touched by someone who is ill with diarrhoea and vomiting 
  • cross-contamination (the spread of bacteria, such as E. coli, from contaminated foods)


Cross-contamination is a cause of food poisoning that is often overlooked. It occurs when harmful bacteria are spread between food, surfaces and equipment.

For example, if you prepare raw chicken on a chopping board and do not wash the board before preparing food that won't be cooked (such as salad) harmful bacteria can be spread from the chopping board to the food.

Cross-contamination can also occur if raw meat is stored above ready-to-eat meals. If juices from the meat drip on to the food below, it can contaminate it.

Sources of contamination

Food contamination is usually caused by bacteria, but it can also sometimes be caused by viruses or parasites. Some common sources of contamination are described below.


In the UK, campylobacter is the most common bacterial cause of food poisoning.

Campylobacter bacteria are usually found on raw or undercooked meat (particularly poultry), unpasteurised milk and untreated water. Undercooked chicken liver and liver pâté are also common sources.


Salmonella bacteria are often found in raw meat and poultry. They can also be passed into dairy products such as eggs and unpasteurised milk.


Listeria bacteria may be found in a range of chilled, ready-to-eat food, including:

  • pre-packed sandwiches
  • pâté
  • butter
  • soft cheeses, such as brie, camembert or others with a similar rind
  • soft blue cheese
  • cooked sliced meats
  • smoked salmon

With all of these foods it is important they are eaten by their 'use by' dates.

Read more about listeriosis

Escherichia coli (E. coli)

Escherichia coli, often known as E. coli, are bacteria found in the digestive systems of many animals, including humans. Most strains are harmless but some strains can cause serious illness.

Most cases of E. coli food poisoning occur after eating undercooked beef (particularly mince, burgers and meatballs) or drinking unpasteurised milk.


The virus that most commonly causes gastrointestinal illness is the norovirus. It is easily transmitted from person to person, from contaminated food or water.

Raw shellfish, particularly oysters can be a source of viral contamination. A study funded by the Foods Standards Agency (FSA) found that three-quarters of oysters sampled from harvesting beds within UK waters contained norovirus, although in half of these it was only detected at low levels.

Currently, these findings do not provide any greater indication of the risk of becoming ill at the point where oysters are purchased and consumed.

The FSA advises that older people, pregnant women, very young children and people who are unwell should avoid eating raw or lightly cooked shellfish to reduce their risk of getting food poisoning.


In the UK, food poisoning from parasites is rare. It is much more common in the developing world.

Toxoplasmosis is the most likely cause of parasitical food poisoning in the UK. It is caused by a parasite that is found in the digestive systems of many animals, particularly cats.

Humans can get toxoplasmosis by consuming undercooked contaminated meat or food or water contaminated with the faeces of infected cats.

Read more about toxoplasmosis.

Medical Review: March 27, 2013
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