This snapshot, taken on
, shows web content acquired for preservation by The National Archives. External links, forms and search may not work in archived websites and contact details are likely to be out of date.
The UK Government Web Archive does not use cookies but some may be left in your browser from archived websites.
Text only

Food bugs

bugs Some types of bacteria in food can give us food poisoning. Check out how this happens and which bugs cause which symptoms.

What happens in the body

When you eat food containing certain types of bacteria, these bacteria – or the chemicals they produce – can give you food poisoning.

When you swallow the bacteria you will start to see symptoms after a delay called the ‘incubation period’. This delay is because most bacteria that cause food poisoning need time to multiply in the intestine.

The length of the incubation period depends on the type of bacteria and how many have been swallowed. It could be as little as a few hours or as much as several days.

Because the bacteria enter the body through the digestive system, generally this is the part of the body where the symptoms appear. So you might get nausea, vomiting, abdominal cramps and diarrhoea, or a combination of these. In some cases, food poisoning can cause very serious illness or even death.

When the bacteria get into your body, they stick to the lining of the intestine and destroy the cells there, either by sheer weight of numbers or by the toxins (poisons) they produce. Sometimes these toxins are absorbed and cause damage elsewhere in the body.

Some bacteria produce toxins when they grow in food. Because the toxins themselves are harmful, the bacteria don't need to multiply in the intestine to make you ill, so the symptoms come on very quickly.

How bacteria grow

Bacteria need warmth and moisture to grow – that’s why we need to keep certain foods nice and cold in the fridge. Bacteria reproduce by dividing themselves, so one bacterium becomes two, two become four, and so on.

In the right conditions, one bacterium could become several million in eight hours and thousands of million in twelve hours.

This means that if a food is contaminated with a small number of bacteria and you leave it out of the fridge overnight, it could be seriously contaminated by the next day. Then just one mouthful could make you ill. If you put food in the fridge it will stop bacteria from multiplying.

Since you can't see, taste or smell bacteria, the only way you can be sure that food is safe is to follow good food hygiene at all times.

About the bugs – campylobacter, clostridium, salmonella, listeria and E. coli


Campylobacter is the most common identified cause of food poisoning. It’s found mainly in poultry, red meat, unpasteurised milk and untreated water.

Although it doesn't grow in food, it spreads easily, so if you’re not careful only a few bacteria in a piece of raw chicken could spread onto food that is ready-to-eat and cause food poisoning.

So make sure you keep raw meat separate from ready-to-eat food and always cook food properly to kill any bacteria that might be in it.

What are the symptoms?
  • diarrhoea, which can be severe and bloody, with abdominal cramps
  • vomiting is very rare

Clostridium perfringens

Clostridium perfringens is found at low levels in many types of food, particularly meat and poultry, and products made out of them. It’s also found in the soil, people’s intestines and animal intestines, in sewage and in animal manure.

Unlike many other types of bacteria that cause foodborne disease, Clostridium perfringens isn't completely destroyed by ordinary cooking. This is because it produces spores that can resist heat.

The bacteria are killed by cooking, but the heat-resistant spores they produce can survive and the heat might actually make the bacteria grow. If the food isn’t eaten at once, but is allowed to cool slowly, the bacteria that are produced when the spores germinate multiply rapidly.

Unless the food is reheated so that it’s steaming hot (at least to 60°C and preferably to 75°C), the bacteria will survive.

After the bacteria have been eaten, if there are enough of them, the bacteria will produce toxins and the toxins will cause symptoms.

Foods most likely to be associated with Clostridium perfringens food poisoning are those that are cooked slowly in large quantities and left to stand for a long time at room temperature.

So make sure you cool food as quickly as possible (ideally within one to two hours) and store it in the fridge. You can help food cool down more quickly by dividing it into smaller portions.

What are the symptoms?
  • diarrhoea and severe abdominal pain
  • occasionally causes nausea
  • vomiting or fever are rare


Salmonella is the second-most common cause of food poisoning after campylobacter. It has been found in unpasteurised milk, eggs, products containing raw egg, meat and poultry. It can survive if food isn’t cooked properly.

Salmonella can grow in food – unless the food is chilled. There only need to be a small number of bacteria in a food for them to multiply.

People infected with salmonella should be particularly careful with personal hygiene because they could infect anyone who comes into direct contact with them. For example, if someone with salmonella doesn't wash their hands properly after going to the toilet, they could have bacteria on their hands.

What are the symptoms?
  • diarrhoea
  • vomiting
  • fever
  • abdominal pain


Listeria or Listeria monocytogenes can cause illness in certain groups of people, such as pregnant women, unborn and newborn babies, and anyone with ‘reduced immunity’ particularly the over 60s.

People with reduced immunity include people who've had transplants; are taking drugs that weaken the immune system; or with cancers affecting their immune system, such as leukaemia or lymphoma. Among these vulnerable groups, the illness is often severe and can be life-threatening.

Listeria has been found in certain chilled ready-to-eat foods, such as pre-packed sandwiches, butter, cooked sliced meats, smoked salmon, soft cheeses and pâtés.

Anyone in these vulnerable groups should avoid eating soft cheeses such as Camembert, Brie or chevre (a type of goats' cheese), or other cheeses that have a similar rind (whether they’re pasteurised or unpasteurised), soft blue cheeses, and all types of pâté, including vegetable pâté.

Take special care to follow the storage instructions on food labels. Your fridge should be between 0°C and 5°C – and chilled foods should be kept out of the fridge for the shortest time possible. Don’t use food after its 'use by' date.

What are the symptoms?
  • flu-like symptoms (high temperature, muscle pain and can include nausea and diarrhoea)
  • severe cases can cause blood poisoning and meningitis, symptoms include severe headaches, stiff neck, confusion, fits, lack of physical co-ordination
  • can cause spontaneous abortion or stillbirth in pregnant women

E. coli

Most strains of E. coli are harmless, but the strain called E. coli O157 can cause severe illness. This is because it can produce toxins (called verocytotoxins). In other countries different strains that produce these toxins are more common, such as E. coli O111 and E. coli O26.

E. coli O157 (and other similar toxin-producing E. coli) are transmitted through eating, drinking or contact with undercooked minced beef and milk that is unpasteurised, hasn’t been pasteurised properly, or has been contaminated after pasteurisation.

It's also possible to become infected by direct contact with people or animals that are infected, or with land contaminated with animal faeces.

What are the symptoms?
  • Bloody diarrhoea and abdominal cramps
  • Can have very serious complications, including kidney failure, severe anaemia and neurological problems
  • Can sometimes lead to death