Gastroenteritis is an infection of the stomach and intestines (bowel). The infection interferes with one of the main functions of the intestines, which is the absorption of water from the contents of your intestines and into the body.

This is the reason why the most common symptom of gastroenteritis is watery diarrhoea, and why dehydration is such a common complication.

The most two most common causes of gastroenteritis in adults in England are:

• a norovirus infection
• food poisoning caused by bacteria

Noroviruses

Noroviruses are the most common cause of viral gastroenteritis in adults. Norovirus infections are sometimes called 'winter vomiting disease' because people tend to get them during the winter months. However, norovirus infections can occur at any time of the year.

Norovirus outbreaks are common, particularly within contained environments such as hospitals, nursing homes, schools and cruise ships. This is because the illness spreads very easily from person to person, and the virus can survive for several days in a contaminated area.

Noroviruses can be spread through contact with an infected person, through contact with contaminated surfaces or objects, or by eating or drinking contaminated food or water.

There are many different types of norovirus, and it is possible for a norovirus infection to occur several times. This is because after getting the illness, immunity to the virus only lasts for 14 weeks.

For more information, take a look at the Health A-Z topic on norovirus infections.

Food poisoning

Most cases of bacterial gastroenteritis are caused by food poisoning.

Some cases of viral gastroenteritis are also caused by food poisoning. Food can be contaminated with a virus if handled by a person with a viral infection.

Food can become contaminated at any stage during its production, processing or cooking. For example, it can be caused by:

• not cooking food at the right temperature and/or for the right length of time
• not chilling food at the correct temperature
• the food being handled by someone who has not washed their hands properly
• people eating food after it has passed its 'use by' date
• cross-contamination

For more information, see the Health A-Z topic on food poisoning.

Cross-contamination

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 then do not wash the board before preparing a ready-to-eat meal such as a salad or sandwiches, harmful bacteria can be spread from the chopping board to the ready-to-eat meal.

Cross-contamination can also occur if you store raw meat above ready-to-eat meals. The meat juices can drip onto the meals and contaminate them.

The most common types of bacteria associated with gastroenteritis are:

• campylobacter - a bacterium found in raw meat and poultry, unpasteurised milk and untreated water
• salmonella - a bacterium found in raw meat, poultry, eggs and unpasteurised milk
• escherichia coli (E. coli) - a bacterium found in undercooked beef and unpasteurised milk

Travellers' diarrhoea

Travellers' diarrhoea is a term used to refer to gastroenteritis that is acquired when travelling abroad. Travellers' diarrhoea can be caused by a range of different bacteria or parasites such as:

• the shigella bacterium or the entamoeba parasite, which are both spread through poor hygiene and cause a type of travellers' diarrhoea known as dysentery
• cryptosporidium, which is a parasite found in soil, food or water that has been contaminated with animal or human faeces
• giardia intestinalis, which is a parasite found in water that has been contaminated with animal or human faeces (infections caused by this parasite are known as giardiasis)

For more information about these, see the Health A-Z topic on travellers' diarrhoea.