30.MAR.2017 6 MIN READ | 6 MIN READ

If you’ve ever had an episode of food poisoning, you know how dreadful it can be – common symptoms include nausea, vomiting, diarrhoea, headache, fever and stomach pain.

Last updated on 7 May 2021

In most cases, people get better within a day or two – but in some cases, depending on its cause, delayed diagnosis and treatment can lead to severe health consequences.

Causes of food poisoning

Food poisoning causes

Food poisoning, otherwise known as gastroenteritis, is an illness caused by consuming contaminated food. It is usually not life-threatening and most people get better within a few days without medical treatment. This could be food contaminated by bacteria such as salmonella or Escherichia coli (E. coli), viruses such as the norovirus or parasites.

The food production chain involves several steps including, production, processing, distribution and preparation. Contamination of food can occur at any point along this process. Here are some examples of contamination at these different stages:


  • The yolk of an egg can be contaminated if a hen’s reproductive organs are infected.
  • Fish in some tropical reefs may acquire toxin from the smaller sea creatures they eat.


  • Contamination of fruits or vegetables if contaminated water or ice is used to wash, pack or chill the items.
  • Germs on an animal’s hide originating from the intestines getting into the final meat product during the slaughter process.


Food poisoning and distribution

  • Refrigerated food left on a loading dock for long time in warm weather could reach temperatures that allow bacteria to grow.
  • Fresh produce can be contaminated if it is loaded into a truck that was not cleaned after transporting live animals or animal products.


  • Using the same knife or cutting board without washing to cut raw meat and fruits and vegetables.
  • An ill food worker who does not wash their hands carefully can spread germs during food handling.

Types of food poisoning

Here are 5 bacteria commonly responsible for causing food poisoning, and the food they can be found in.


There are hundreds of different types of salmonella and not all of them are harmful to humans.

Salmonella bacteria thrive in:

  • Poultry or meat contaminated by poor food handling
  • Dairy products, including eggs with dirty shells, and unpasteurised milk and cheese
  • Juice, fruits, vegetables, spices and nuts


Campylobacter is the most common identified cause of food-borne disease. Although it doesn’t grow in food, it spreads easily so a few bacteria in a piece of undercooked chicken could cause illness.

Campylobacter bacteria thrive in:

  • Poultry, red meat
  • Unpasteurised milk
  • Untreated water

E. coli

E. coli is the name of a type of bacteria that lives in your intestines and in the intestines of animals. Although most types of E. coli are harmless, some types can make you sick.

E. coli bacteria thrive in:

  • Beef contaminated with faeces
  • Unpasteurised milk and apple cider, alfalfa sprouts
  • Contaminated water


Listeria is a bacteria present in most animals and humans. The bacteria is normally not harmful if its host is healthy. These bacteria are cold-tolerant, although growth is slow below 5°C. When eaten, it colonises the gut and then spreads to other parts of the body.

Listeria bacteria thrive in:

  • Raw milk and foods made from raw milk
  • Processed meats (eg. hot dogs, luncheon meat)
  • Unwashed raw produce

Clostridium perfringens

Shortened as C. perfringens, the infection happens most often in foods that are prepared in large quantities and then kept warm for a long time before serving. Thorough cooking may kill the bacteria but not necessarily the spores that can grow into new cells. If cooked food is not served promptly, the spores may develop into new cells.

C. perfringens bacteria thrive in:

  • Reheated gravies
  • Beef
  • Poultry

Signs and symptoms of food poisoning

Food poisoning symptoms

Symptoms of food poisoning vary according to the source of infection. First symptoms may appear as soon as 1 hour or as long as 28 days after consumption of contaminated food. Common cases of food poisoning will typically include at least three of the following symptoms:

  • Abdominal cramps
  • Diarrhoea
  • Vomiting
  • Loss of appetite
  • Mild fever
  • Weakness
  • Nausea
  • Headaches

What to do if I have food poisoning

If we are healthy, our immune systems can deal with small numbers of bacteria and viruses. At higher levels, they can make us quite sick. Most cases of food poisoning are mild and the symptoms clear up in a few days. During this period of recovery, the goal is to prevent dehydration. Drink lots of liquids, but avoid milk and caffeinated beverages.

If you developed the symptoms after returning from a trip, you should consider seeing a doctor, as the cause of the food poisoning might be a bacterial infection, which is more commonly related to travel.

When to seek emergency medical attention

Although most food poisoning cases are mild, some can be serious and life-threatening. See your doctor immediately if you have:

  • Diarrhoea persisting for more than 3 days
  • A high fever (39°C and above)
  • Severe or persistent abdominal pain
  • Persistent vomiting for more than 12 hours
  • Symptoms of severe dehydration, eg. a dry mouth, passing little to no urine or having urine that’s darker than usual, dizziness, and difficulty keeping fluids down
  • Blood in your stool
  • Breathing difficulty

Those who are vulnerable, such as pregnant women, the elderly or people with poor immune systems can get very ill from food poisoning. Some cases may even be life-threatening. If you experience any of these symptoms, belong to the above-mentioned vulnerable groups, or have other serious underlying medical problems, you should contact your doctor immediately.


Reviewed by Dr Kelvin Thia, gastroenterologist at Mount Elizabeth Hospital


How Food Gets Contaminated – The Food Production Chain. (2017, September 05) Retrieved April 06, 2021, from https://www.cdc.gov/foodsafety/production-chain.html

Food Poisoning. (2019, March 07) Retrieved April 06, 2021, from https://www.healthline.com/health/food-poisoning

Thia Teck Joo Kelvin
Mount Elizabeth Hospital

Dr Kelvin Thia is a gastroenterologist practising at Mount Elizabeth Hospital, Singapore.