Gastroenteritis is usually caused by an infection of the stomach and intestines.

The infection interferes with the absorption of water from the contents of your intestines into the body, which is why watery diarrhoea is the most common symptom of gastroenteritis and why dehydration can occur.

In the UK, the two most common causes of gastroenteritis in adults are a norovirus infection and bacterial food poisoning.

Norovirus

Norovirus is the most common cause of viral gastroenteritis in adults. It is sometimes referred to as the "winter vomiting bug" because it tends to be more widespread during the winter months. However, infections can occur at any time of the year.

The virus is passed out in the stools (faeces) of someone with the infection and, if the person does not wash their hands after going to the toilet, the virus can be transferred to any surfaces, objects and food they touch, where it can survive for several days.

The infection can then be passed to someone else who eats contaminated food or touches a contaminated object or surface and then touches their mouth.

Small droplets of infected faeces or vomit can also be carried in the air, which others can breathe in.

Norovirus infections are easily spread in these ways, particularly in confined environments, such as hospitals, nursing homes, schools and cruise ships.

There are many different types of norovirus and it is possible for you to get a norovirus infection several times. This is because any immunity to the infection you develop after being ill only lasts a few months.

Read more about norovirus infections.

Food poisoning

Most bacterial infections that cause gastroenteritis are the result of food poisoning.

Contamination with bacteria can occur at any stage during the food's production, processing or cooking. For example, food poisoning can be caused by:

• not cooking food at the right temperature or for the right length of time
• not chilling food at the correct temperature
• someone who has not washed their hands properly handling the food 
• eating food after it has reached its use-by date
• cross-contamination (when harmful bacteria is spread between food, surfaces and equipment)

The most common types of bacteria that are associated with gastroenteritis are called campylobacter, salmonella and escherichia coli (E. coli). These are generally found in raw or undercooked meat, unpasteurised milk and untreated water.

Read more about food poisoning.

Travel infections

Travellers to areas with poor levels of sanitation and water hygiene are also at risk of developing gastroenteritis. This is often known as "traveller's diarrhoea".

Traveller's diarrhoea can be caused by a range of bacteria, viruses or parasites, often similar to those infections acquired through food poisoning in the UK. Other causes include:

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

Read more about preventing traveller's diarrhoea.