Food can become contaminated at any stage during its production, processing or cooking.
For example, food poisoning 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
- someone with unclean hands touching the food
- eating the food after it has passed its 'use by' date
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 do not wash the board before preparing a ready-to-eat meal such as a salad, harmful bacteria can be spread from the chopping board to the salad.
Cross-contamination can also occur if you store raw meat above ready-to-eat meals. The meat juices can drip on to the meals and contaminate them.
Sources of contamination
Food contamination is usually caused by bacteria. Some common types are described below as are viruses and toxins.
Campylobacter is the most common cause of food poisoning in Ireland. Campylobacter bacteria are usually found in raw meat and poultry, unpasteurised milk and untreated water.
Salmonella bacteria are often found in raw meat and poultry. They can also be passed into dairy products such as eggs and unpasteurised milk.
Listeria bacteria may be found in a range of chilled ready-to-eat food including:
- pre-packed sandwiches
- soft cheeses, such as brie, camembert or others with a similar rind
- soft blue cheese
- cooked sliced meats
- smoked salmon
Escherichia coli, known as E. coli, are bacteria found in the digestive system of many animals, including humans. Most strains are harmless but some strains can cause serious illness.
Some cases of E. coli food poisoning occur after eating undercooked beef or drinking unpasteurised milk.
Two viruses that commonly cause food poisoning are the rotavirus and the norovirus.
The rotavirus is more common in children than adults because most adults develop immunity (resistance) to it. Noroviruses can affect people of any age.
People infected with either virus can contaminate food if they do not wash their hands properly after going to the toilet and then handle food.
In Ireland, food poisoning from parasites is rare. It is much more common in the developing world.
Toxoplasmosis is the most likely cause of parasitical food poisoning in Ireland. It is caused by a parasite that is found in the digestive systems of many animals, particularly cats.
People can develop toxoplasmosis by consuming undercooked contaminated meat, or food or water contaminated with the faeces of infected cats.
There is a small risk that oily fish could be contaminated by toxins such as a chemical called polychlorinated biphenyls (PCBs).
The levels of toxins in contaminated fish are thought to be very low, but they could still pose a risk to unborn babies. Therefore pregnant women are advised not to eat more than two portions of oily fish a week.
Examples of oily fish include:
- fresh tuna (not canned tuna, which does not count as oily fish)
Some fish contain a high level of mercury, which can also damage an unborn baby's nervous system. Therefore, pregnant women should avoid eating:
- more than two tuna steaks a week (or four medium cans of tuna a week)