Cross-contamination can occur, 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), as the harmful bacteria can be spread from the chopping board to the salad.
It can also occur if raw meat is stored above ready-to-eat meals and juices from the meat drip on to the food below.
See preventing food poisoning for information about reducing these risks
Types of infection
Food contamination is usually caused by bacteria, but it can also sometimes be caused by viruses or parasites. Some of the main sources of contamination are described below.
Campylobacter
In the UK, campylobacter bacteria are the most common cause of food poisoning. The bacteria are usually found on raw or undercooked meat (particularly poultry), unpasteurised milk and untreated water.
The incubation period (the time between eating contaminated food and the start of symptoms) for food poisoning caused by campylobacter is usually between two and five days. The symptoms usually last less than a week.
Salmonella
Salmonella bacteria are often found in raw or undercooked meat, raw eggs, milk, and other dairy products.
The incubation period is usually between 12 and 72 hours. The symptoms usually last around four to seven days.
Listeria
Listeria bacteria may be found in a range of chilled, "ready-to-eat" foods, including pre-packed sandwiches, cooked sliced meats and pâté, and soft cheeses (such as Brie or Camembert).
All of these foods should be eaten by their "use-by" dates. This is particularly important for pregnant women, because a listeria infection (known as listeriosis) in pregnancy can cause pregnancy and birth complications, and can result in miscarriage.
The incubation period can vary considerably, from a few days to several weeks. The symptoms will usually pass within three days.
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 typically one to eight days. The symptoms usually last for a few days or weeks.
Shigella
Shigella bacteria can contaminate any food that has been washed in contaminated water.
Symptoms typically develop within seven days of eating contaminated food and last for up to a week.
An infection caused by Shigella bacteria is known as bacillary dysentery or shigellosis. See the topic on dysentery for more information about it.
Viruses
The virus that most commonly causes diarrhoea and vomiting is the norovirus. It's easily spread from person to person, through contaminated food or water. Raw shellfish, particularly oysters, can also be a source of infection.
The incubation period typically lasts 24-48 hours and the symptoms usually pass in a couple of days.
In young children, the rotavirus is a common cause of infection from contaminated food. The symptoms usually develop within a week and pass in around five to seven days.
Parasites
In the UK, food poisoning caused by parasites is rare. It's much more common in the developing world.
Parasitic infections that can be spread in contaminated food include:
- giardiasis – an infection caused by a parasite called Giardia intestinalis
- cryptosporidiosis – an infection caused by a parasite called Cryptosporidium
- ameobiasis – a type of dysentery caused by a single-cell parasite (ameoba) called Entamoeba histolytica (this is very rare in the UK)
The symptoms of food poisoning caused by a parasite usually develop within 10 days of eating contaminated food, although sometimes it may be weeks before you feel unwell.
Salmonella infection (also known as salmonellosis) is an infection of the digestive tract (or gut), caused by Salmonella bacteria. There are thousands of Salmonella types and they are found in many domestic and wild animals.
How do you get Salmonella infection?
Salmonella bacteria can infect both people and animals, and can also grow in food and the environment.
You become infected with Salmonella by ingesting the bacteria through your mouth. This can be by:
- eating contaminated, undercooked meat (most commonly chicken)
- eating contaminated raw or undercooked eggs
- eating food that contains raw egg butter
- eating food that has been cross-contaminated with Salmonella from raw foods (especially meats, poultry and eggs)
- handling pets, other animals, raw meat and pet meat
- drinking water that has been contaminated, usually by animals or sewage.
Person-to-person spread can happen if you come into contact with microscopic amounts of faeces (poo) from an ill person. Such spread may occur directly by close personal contact, or indirectly by touching contaminated surfaces such as taps, toilet flush buttons, toys and nappies.
What are the signs and symptoms?
Symptoms usually start between 6 hours to 7 days (usually 1 to 3 days) after you have ingested the bacteria and typically last for between 2 to 7 days.
Symptoms can include:
- diarrhoea
- stomach cramps
- nausea and vomiting
- fever
- headache.
Salmonella typhi and Salmonella paratyphi are certain types of Salmonella bacteria that cause typhoid and paratyphoid fever, which affect the whole body, not just the digestive tract.
How do you know if you have it?
There are many causes of gastroenteritis, and laboratory testing of a faecal specimen is necessary to confirm that symptoms are due to Salmonella infection.
People with confirmed or suspected Salmonella infection should:
- Drink plenty of fluids such as water or oral rehydration drinks (available from pharmacies) to avoid dehydration. Dehydration is especially dangerous for babies and the elderly.
- Avoid anti-vomiting or anti-diarrhoeal medications unless prescribed or recommended by a doctor.
People with typhoid or paratyphoid fever will require antibiotic treatment.
If you experience severe or prolonged symptoms you should visit a doctor.
What should you do if you have the infection?
- Do not go to work or school for at least 24 hours after symptoms have finished, or 48 hours if you work in or attend a high risk setting, such as health care, residential care or child care, or handle food as part of your job.
- People with typhoid or paratyphoid fever who work in a high risk setting as described above will be contacted by their local public health unit to discuss extra precautions.
- Wash and dry your hands thoroughly after going to the toilet.
- Avoid preparing or handling food and drinks for other people in your household until at least 24 hours after your symptoms have finished. If you must prepare or handle food, thoroughly wash your hands beforehand to reduce the risk of spreading the infection to others.
- Immediately remove and wash using detergent and hot water any clothes or bedding contaminated with vomit or diarrhoea.
- After an episode of diarrhoea or vomiting, clean contaminated surfaces (for example benches, floors and toilets) immediately using detergent and hot water. Then disinfectant surfaces using a bleach-based product diluted according to manufacturer’s instructions.
- Clean carpet or soft furnishings contaminated with diarrhoea or vomit immediately using detergent and hot water and then steam clean.
- Avoid contact with people who have gastroenteritis symptoms.
- Wash and dry your hands thoroughly after going to the toilet, handling animals, changing nappies or cleaning up vomit or diarrhoea, and before eating or drinking. If hand-washing facilities are not available use an alcohol-based gel.
- Raw meats, especially poultry, can contain Salmonella bacteria. Keep raw foods separate from cooked and ready-to-eat foods (for example salads) to prevent cross-contamination. Store raw meat below ready-to-eat food in the refrigerator and use separate chopping boards and knives for raw and ready-to-eat foods.
- Keep cold food below 5 °C and hot food above 60 °C.
- Cook meat and poultry thoroughly to a temperature of 75 °C or until meat juices run clear and are not pink.
- Do not use dirty or cracked eggs.
When travelling
When travelling to developing countries, especially in Asia, the Pacific islands, Africa, the Middle East and Central and South America, you should avoid:
- salads and fresh fruit salads
- raw or cold seafood, including shellfish
- raw or runny eggs
- cold meat
- unpasteurised milk and dairy products (including ice-cream)
- ice in drinks and flavoured ice blocks.
Fruit that you peel yourself is usually safe. Remember – ‘cook it, boil it, peel it, or leave it’.
Use bottled water or disinfect water (by boiling, chemical treatment or purifiers) for drinking and brushing teeth.
Read more about healthy international travel.
Is there a vaccine for Salmonella?
There are no vaccines that protect against most types of Salmonella. However, a typhoid vaccine is available for overseas travellers and can provide some protection. See your doctor or travel medicine specialist at least 2 months prior to departure to see if any vaccinations or medications are recommended.
Where to get help
Remember
- Safe food preparation and thorough cooking, and washing your hands after contact with raw meats and animals can help prevent Salmonella infection.
View and download this information as a PDF factsheet (300KB).
Acknowledgements
Public Health
This publication is provided for education and information purposes only. It is not a substitute for professional medical care. Information about a therapy, service, product or treatment does not imply endorsement and is not intended to replace advice from your healthcare professional. Readers should note that over time currency and completeness of the information may change. All users should seek advice from a qualified healthcare professional for a diagnosis and answers to their medical questions.