Gastrointestinal illness - the kind of illness that makes us feel sick, vomit or have diarrhoea - affects people of all ages and is one of the most common health complaints in the United Kingdom. It is also one of the most common health issues that people experience when travelling abroad.

While diarrhoea and vomiting are typically short-lived, they can be serious for some. Understanding their causes can help you take appropriate steps to protect yourself and your family.

Viral infections

Viruses are one of the most common causes of gastrointestinal illness, spreading easily in environments where individuals have close contact, such as hospitals, care homes, schools and nurseries.

Norovirus is perhaps the most widely recognised cause, commonly known as the ‘winter vomiting bug’, although infections can occur at any point in the year. The virus is mostly spread person to person but can also be spread by contaminated food or water. It is possible to catch norovirus more than once within a short period, as multiple strains circulate at any given time. The incubation period, which is the time between catching an illness and showing symptoms, is typically 12 to 48 hours. Most people recover after a few days’ rest, but vulnerable groups are at risk of complications including dehydration, which can cause hospitalisation.

Rotavirus is the most common cause of gastroenteritis in infants and very young children, producing severe watery diarrhoea alongside vomiting, fever and stomach cramps. Since the introduction of the rotavirus vaccine to the national vaccination schedule in 2013, the incidence among infants has declined considerably. Infants receive 2 doses at 8 and 16 weeks of age.

Adenovirus is a group of viruses that usually cause symptoms like a common cold but can cause various infections including gastroenteritis. It does not follow a seasonal pattern and infections may occur at any time of year. Although most common in infants and young children, individuals of any age may be affected. Symptoms are generally mild, though infections can prove more severe in those with weakened immune systems.

Bacterial infections

Bacteria are another significant cause of gastrointestinal illness, frequently associated with food poisoning. Illness caused by a bacterial infection typically lasts longer than that due to a virus.

Campylobacter are the most common bacterial causes of food poisoning globally. You typically catch the infection from raw or undercooked meat (particularly poultry), unpasteurised milk, or untreated water. Poor hygiene during food preparation can cause it to spread through cross-contamination, and it is also passed on through contact with an infected animal or its environment, for example pets suffering from diarrhoea. The incubation period is generally 2 to 5 days.

Shigella can cause severe diarrhoea (which is sometimes bloody), fever and stomach cramps. Transmission occurs through contact with contaminated poo, either directly from person to person, or indirectly through contaminated food, water or surfaces. Travellers to regions with inadequate sanitation face particular risk, although it is present in the UK. Globally, most cases of shigellosis are in children younger than 5 years of age, but all ages can be affected. There is also a risk of Shigella transmission through sexual contact, which is more likely among men who have sex with men. 

Salmonella may be present in eggs, chicken, pork and dairy products, as well as fruit and vegetables that have come into contact with livestock, manure or untreated water. Symptoms including diarrhoea, stomach cramps, nausea, vomiting and fever typically develop in the 12 to 72 hours following infection. The illness usually persists for 4 to 7 days.

E.coli (Escherichia coli) bacteria are commonly found in the intestines of humans and animals. While many strains exist harmlessly in the digestive system, others may cause infections, including gastrointestinal illness. The bacterium is present in poo and can survive in the environment.

STEC (Shiga toxin-producing Escherichia coli) can cause diarrhoea, often bloody, with abdominal pain. In some cases, particularly in young children and older adults, infection can lead to haemolytic uraemic syndrome (HUS), a serious kidney complication. Spread mostly occurs through contaminated food, water and contact with farm animals. Common food sources may include undercooked minced beef, unpasteurised milk and dairy products, and raw leafy greens.

Parasitic infections

Cryptosporidium is a microscopic parasite responsible for an illness known as cryptosporidiosis. This organism lives in the intestines of infected humans and animals and often spreads through contaminated drinking water and other water sources such as swimming pools, lakes and rivers, as well as through raw milk and vegetables. Children under 5 years are particularly susceptible, with infections often associated with animal contact at petting farms or lambing events.

Treatment and management

Diarrhoea and vomiting can typically be managed at home. The primary concern is ensuring you take in enough fluids to prevent dehydration.

Recommended actions:

• remain at home and obtain sufficient rest
• consume plenty of fluids such as water or squash, taking small sips if experiencing nausea
• continue breast or bottle feeding infants, offering smaller feeds more frequently if vomiting occurs
• eat when able, avoiding fatty or spicy foods
• take paracetamol for discomfort if required

Actions to avoid:

• consuming fruit juice or fizzy drinks, which may exacerbate diarrhoea
• administering anti-diarrhoeal medication to children under 12 years
• giving aspirin to children under 16 years

Diarrhoea typically resolves within 5 to 7 days, while vomiting usually stops within 1 to 2 days. Seek healthcare advice if symptoms continue after this or you experience severe illness.

Individuals should consult a pharmacist if they or their child show signs of dehydration, such as dark-coloured urine or reduced urination frequency. Pharmacists may recommend oral rehydration solutions or, for adults, temporary anti-diarrhoeal medication.

How to prevent spreading a stomach bug

To prevent transmission of a viral or bacterial infection:

• wash hands frequently and thoroughly with soap and warm water. Don’t rely on alcohol gels alone as they don’t kill norovirus
• clean surfaces regularly with bleach-based disinfectant
• stay off school or work until symptom-free for at least 2 days
• do not visit others in hospitals or care homes until 2 days after symptoms stop
• swimming pools should be avoided for a minimum of 48 hours following the end of symptoms. In the case of cryptosporidium swimming pool use should be avoided for 2 weeks after symptoms

• Are a food handler whose work is connected with the preparation or handling of food and drink.
• Are a health care or nursery member of staff or other staff who has direct contact or contact through serving food, with highly susceptible patients or person to whom food poisoning would have particularly serious consequences.

It may not be due to your last meal. Very often, people suffering from suspected food poisoning feel sure that the cause is the last meal eaten, especially if this happens to be a meal at a restaurant or takeaway.

Food poisoning bacteria take quite a long time before actually making you ill. Usually between 12 -48 hours but sometimes longer. So it is more likely that the cause of the illness is something eaten the previous day or even 2-3 days earlier. Some illnesses can take up to 11 days before you show any symptoms.

• Diarrhoea
• Vomiting (sickness)
• Nausea
• Fever
• Headaches
• Stomach pains

• Food prepared too far in advance and then kept at room temperature. Food poisoning bacteria grow rapidly at room temperature. Any food which has been prepared in advance must therefore be cooled as quickly as possible and refrigerated in order to slow bacterial growth.
• Undercooking of food. This may be dangerous as any harmful bacteria in the food will not be destroyed.
• Not reheating food to high enough temperatures. Reheated products are those that have been previously cooked, allowed to cool and then reheated before consumption. If the cooling has not been rapid enough, any spores in the food will have had time to germinate. It is vital that these products are thoroughly re-cooked in order to destroy any bacteria that may have been produced by the spores
• Cross contamination from raw food to cooked food. Food poisoning bacteria may be naturally present in raw food, especially raw meat and poultry. If these bacteria are allowed to contaminate food that is going to be eaten without receiving further heat treatment, food poisoning can result. Cross contamination may be direct, for example poor storage when the juices from raw meat are allowed to drip on to cooked food, or indirect for example via a chopping board, work surface dirty dishcloths or the food handlers hands.
• Failure to keep hot food above 63ºC. Cooking destroys bacteria but not their spores, therefore if hot foods are held below 63ºC the spores may germinate and produce food poisoning bacteria.
• Poor personal hygiene and infected food handlers. Poor personal hygiene can result in food becoming contaminated with bacteria. Additionally, persons suffering from infections, e.g. vomiting, diarrhoea and infected cuts can contaminate the food. It is important to remember that anyone who has been in contact with someone suffering from food poisoning can pass on bacteria, even though they may show no symptoms themselves.

Reports of food poisoning reach us by different routes. Sometimes people have seen their GP who then reports the illness to us. Doctors are required by law to report either confirmed or suspected food poisoning. Affected persons may also notify us directly without first having seen their GP. We will respond to all cases by contacting the affected person and asking for a lot of details about the illness, including the dates and times of symptoms, a food history, your occupation and details of people in your household. This information can be vital in preventing further spread of the infection. You are likely to also be asked to provide a stool specimen if you have not already done so.

If there is evidence that your illness is related to a specific restaurant or takeaway in the area then we will want to discuss the matter in detail with you.

In most cases it is very difficult to connect an event of illness to a particular business. Very often the food eaten will have been destroyed by the time your illness shows its symptoms, no stool samples are provided to actually confirm food poisoning, or the incident is reported after the person is well so a sample would be pointless.

Of course if a whole group of people at a party or wedding reception for example are all affected by the same symptoms at the same time then circumstantial evidence may be all that is needed to take the matter further.