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

Cross-contamination is a cause of food poisoning that’s 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 don’t 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 become contaminated.

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.

Campylobacter

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.

The incubation period (the time between eating contaminated food and the onset of symptoms) for food poisoning caused by campylobacter is 2-5 days.

Salmonella

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

The incubation period for food poisoning caused by salmonella is 12-48 hours.

Listeria

Listeria bacteria may be found in a range of chilled, ready-to-eat foods 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

It’s important that all of these foods are eaten by their ‘use-by’ dates.

The incubation period for food poisoning caused by listeria can vary considerably, from two days to three months.

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 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 incubation period for food poisoning caused by E. coli is usually 3-4 days, but it’s possible for symptoms to take longer to appear (up to two weeks).

Viruses

The virus that most commonly causes gastrointestinal illness is the norovirus. It’s easily spread 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 don’t provide any greater indication of the risk of becoming ill at the point where oysters are purchased and consumed.

However, 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.

Parasites

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

Toxoplasmosis is the most likely cause of parasitical food poisoning in the UK. It’s caused by a parasite that’s 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.

Food poisoning is an illness caused by eating contaminated food. Most people get better without the need for treatment.

In most cases, food that causes food poisoning is contaminated by bacteria, such as salmonella or Escherichia coli (E. coli), or a virus, such as the norovirus.

The symptoms of food poisoning usually begin 1-3 days after eating contaminated food. They include:

• feeling sick
• vomiting
• diarrhoea 
• stomach cramps

Some toxins can cause food poisoning within a much shorter time. In these cases, vomiting is the main symptom.

Foods particularly susceptible to contamination if not handled, stored or cooked properly include:

• raw meat and poultry
• raw eggs
• raw shellfish
• unpasteurised milk
• ‘ready to eat’ foods, such as cooked sliced meats, pâté, soft cheeses and pre-packed sandwiches

How is food contaminated?

Food can become contaminated at any stage during production, processing or cooking. For example, food poisoning can be caused 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
• leaving cooked food for too long at warm temperatures  
• someone who is ill or who has dirty hands touching the food
• eating food that has passed its ‘use by’ date
• cross-contamination (the spread of bacteria, such as E. coli, from contaminated foods)

Read more about the causes of food poisoning.

Treating food poisoning

Most people with food poisoning get better without the need for treatment. 

To help relieve your symptoms you should rest and drink plenty of fluids. It is best to avoid food until you feel much better. When you start eating again, choose foods that are easily digested, such as toast.

It’s important that you do not become dehydrated because it will make you feel worse and lengthen your recovery.

Try to drink as much water as you can, even if you can only sip it, particularly every time you pass diarrhoea.

Oral rehydration salts (ORSs) are recommended for people vulnerable to the effects of dehydration, such as the elderly and those with another health condition (see below).

ORSs help replace salt, glucose and other important minerals lost through dehydration. They are available in sachets from pharmacies and you dissolve them in water to drink.

Read more about treating food poisoning.

When to see your GP

It’s not usually necessary to see your GP if you have food poisoning. You only need to see them if:

• your symptoms are severe and do not improve after a few days
• you have a high temperature (fever) of 38°C (100.4°F) or over 
• you have symptoms of severe dehydration, such as sunken eyes and passing small quantities of dark, strong smelling urine
• there has been an outbreak of similar cases of food poisoning linked to a possible source of contamination
• you have a baby with food poisoning

Occasionally, food poisoning can have more serious effects on a person’s health, particularly if they are vulnerable to infection. For example, if you are over 65 years of age, or you have a condition that weakens your immune system, such as HIV or cancer, your risk of developing more serious symptoms is increased. Babies are also at increased risk.

Signs that you may have a more serious case of food poisoning that requires medical attention include:

• vomiting that lasts for more than two days
• not being able to keep liquids down for more than a day
• diarrhoea that lasts for more than three days or is bloody
• fever 

Read more about when to seek medical advice for food poisoning.

The best way to avoid getting food poisoning is to ensure you maintain high standards of food hygiene when storing, handling and preparing food.

According to the Food Standards Agency (FSA), a useful way of preventing food poisoning is to remember the four Cs:

• cleaning
• cooking
• chilling
• cross-contamination (avoiding it)

The FSA also recommends sticking to a food’s ‘use by’ date and the storage instructions on the packet.

Cleaning

You can prevent the spread of harmful bacteria and viruses by maintaining good personal hygiene standards and keeping work surfaces and utensils clean.

Regularly wash your hands with soap and warm water, particularly:

• after going to the toilet
• after handling raw food
• before preparing food
• after touching bins
• after touching pets

You should never handle food if:

• you are ill with stomach problems, such as diarrhoea or vomiting
• you have sores and cuts (unless they are covered with a waterproof dressing)

Cooking

It is important to cook food thoroughly, particularly poultry, pork, burgers, sausages and kebabs. This will kill any harmful bacteria that may be present, such as listeria and salmonella.

Make sure the food is cooked thoroughly and is steaming hot in the middle. To check that meat is cooked, insert a knife into the thickest or deepest part. It is fully cooked if the juices are clear and there is no pink or red meat. Some meat, such as steaks and joints (but not rolled joints) of beef or lamb, can be served rare (not cooked in the middle), as long as the outside has been cooked properly.

When reheating food, make sure it is steaming hot all the way through. Do not reheat food more than once.

Chilling

Certain foods need to be kept at the correct temperature to prevent harmful bacteria from growing and multiplying. Always check the storage instructions on the label.

If food has to be refrigerated, set your fridge to 0-5°C (32-41°F).

If food that needs to be chilled is left at room temperature, bacteria can grow and multiply to dangerous levels.

Cooked leftovers should be cooled quickly, ideally within 1-2 hours, and put in your fridge or freezer. Dividing food into smaller amounts and putting it into shallow containers will speed up the cooling process.

Cross-contamination

Cross-contamination occurs when bacteria are transferred from foods (usually raw foods) to other foods. Contamination can be:

• direct – where one food touches or drips onto another food
• indirect – where bacteria on your hands, work surfaces, equipment or utensils are spread to food

To prevent cross-contamination:

• always wash your hands after handling raw food
• store raw and ready-to-eat foods separately
• store raw meat in sealable containers at the bottom of your fridge so that it cannot drip onto other foods
• use a different chopping board for raw food and ready-to-eat food, or wash it thoroughly in between preparing different types of food
• clean knives and other utensils thoroughly after using them with raw food
• do not wash raw meat or poultry – any harmful bacteria will be killed by thorough cooking, and washing may splash harmful bacteria around the kitchen

The length of time it takes for the symptoms of food poisoning to develop will vary depending on the type of bacteria involved and how much contaminated food is eaten.

The incubation period (the time between eating contaminated food and the onset of symptoms) can be as short as a few hours or as long as several weeks.

Read more about the causes of food poisoning and the differences in incubation periods.

The most common symptoms of food poisoning are:

• nausea
• vomiting 
• diarrhoea

Vomiting usually only lasts for a day or so, but it can sometimes last longer. Diarrhoea will often last for a few days, although you may continue to have an upset stomach for about a week or so.

Other symptoms of food poisoning include:

• stomach cramps
• abdominal pain 
• loss of appetite
• a high temperature of 38°C (100.4°F) or above
• muscle pain
• chills

Most people make a full recovery 1-2 days after the onset of symptoms.

When to seek medical advice

Most cases of food poisoning do not require medical treatment. However, you should seek medical advice if you have any of the following signs or symptoms:

• vomiting that lasts more than two days
• you are unable to keep liquids down for more than a day
• diarrhoea that lasts for more than three days
• blood in your vomit
• blood in your stools
• seizures (fits)
• changes in your mental state, such as confusion
• double vision 
• slurred speech
• signs of severe dehydration, such as a dry mouth, sunken eyes and an inability to pass urine, or passing small amounts of dark, strong-smelling urine

Pregnancy

Always contact your GP if you get food poisoning during pregnancy. Extra precautions may be needed. 

In most cases, food poisoning can be treated at home without seeking medical advice.

It is very important that you do not become dehydrated because it will make you feel worse and slow down your recovery.

Dehydration is a risk because you will lose fluid through vomiting and diarrhoea.

You should try to drink as much water as possible, even if you’re only able to sip it, particularly after you pass diarrhoea.

Oral rehydration salts (ORSs)

Oral rehydration salts (ORSs) are recommended for people vulnerable to the effects of dehydration, such as the elderly and those with a pre-existing health condition.

ORSs are available in sachets from pharmacies. You dissolve them in water to drink and they help replace salt, glucose and other important minerals your body loses through dehydration.

If you have a kidney condition, some types of oral rehydration salts may not be suitable for you. Ask your pharmacist or GP for further advice about this.

Other self care advice

To cope with your symptoms and speed up your recovery you should also:

• rest
• eat when you feel up to it (the gut sometimes needs time to recover and food may cause diarrhoea even if you feel better)
• stick to foods that are easily digested, such as toast, crackers, bananas and rice until you begin to feel better
• avoid alcohol, cigarettes, caffeine and spicy and fatty foods because they will make you feel worse 

Further treatment

Visit your GP or accident and emergency (A&E) department if you are severely dehydrated – for example, if you have sunken eyes and you are unable to urinate.

Your GP may admit you to hospital so that you can be given fluids and nutrients through a tube inserted into a vein (intravenously).

Read more about treating dehydration.

Antibiotics may be prescribed if test results show the source of your food poisoning was bacterial, and your symptoms are severe or last longer than 3-4 days.

Antibiotic tablets are usually used, although you may be given injections if your symptoms are severe or if you are struggling to keep tablets down.