Causes and types of food poisoning

Causes and types of food poisoning

How does food become contaminated?

Food contamination is defined as the result of harmful substances making contact with foodstuffs tainting or spoiling them. When contaminated food is consumed it causes adverse reactions in the body. Contamination typically falls into one of four primary categories. These are:

  • Biological: Contact is made by means of bacteria (pathogens) or toxins, spoiling foodstuffs and resulting in food poisoning when consumed. Such small pathogens split, and each cell then multiplies (in high numbers) in a short space of time. Not all types of bacteria are harmful, but certain types, even as isolated organisms, are known to be particularly toxic to humans while others become so when multiplying in growth inducing conditions. Bacteria exists just about everywhere, including the human body, but is not clearly visible to the naked eye. Other common places wherein bacteria exist and thrive include dust, the air, fabrics (such as kitchen cloths and clothing), as well as raw meat products (high protein foods) and on pets (or pests). Bacteria thrive on food which is high in protein content, stored in water, or subject to oxygen exposure, as well as in foods with a neutral PH level (not acidic or alkaline) and at temperatures between 5ºC and 60ºC (41 ºF and 140 ºF). Conditions bacteria struggle to multiply in include foods which are high in acidity, salt or sugar content, or are dried, canned or vacuum packed.
  • Chemical: This type of contamination results when foodstuffs come into contact with certain chemicals. Chemicals that most often cause contamination on contact include kitchen cleaning products, pesticides and fungicides (most often used on fruits and vegetables which are sold unwashed), those used in ‘non-safe plastics’ (which are then used as food containers), pest control products (like insect sprays or rodent poisons), as well as those used in the oiling of kitchen equipment, like appliances with moving parts (mixers or slicers).
  • Physical: This type of contamination refers to physical contact that is made between foodstuffs and objects such as glass or metal kitchen items / utensils (especially those that are cracked or broken and can harbour bacteria), jewellery, hair, fingernails, dirt particles, and pests (such as cockroaches and rodents). This type of contamination can often occur alongside bacterial contamination, as these organisms can easily attach to such items.
  • Cross-contamination: At any stage of processing, growing, harvesting, storing, shipping or preparing food, cross-contamination can easily happen. This type of contamination is effectively the transfer of harmful organisms (pathogens) from one surface to another (including human contact). Raw, ready-to-eat foodstuffs are particularly vulnerable to contaminants as no cooking, which would help to kill off harmful organisms, takes place.

Pathogens can be found on just about every food source we consume. Illness is most often avoided through the cooking process, which typically kills off harmful organisms before they are ingested.

Foods which are consumed raw are those most vulnerable to contamination as a result. Contact with contaminated water (whether ingested directly or through watered or washed foods) or even faecal matter (whether it be from animals or contact with human sewage) are other common means of transmission of pathogens.

Those involved in food handling and preparation can also easily transfer pathogens during this process. Proper hygiene practices and handwashing between the handling of different foods can help to reduce risk of transmission of a host of germs. Contamination can occur as simply as touching food or using boards and utensils for the preparation of both meat and vegetables, without cleaning these in between (transferring pathogens or organisms between meat and vegetables). Other means of cross-contamination can occur as a result of contact with dirty clothing, coughing or sneezing around food being prepared, handling food in an environment crawling with pests or where waste is stored and not properly sealed, and/or storing food in ways that are not compliant with general safety standards.

Understanding foodborne intoxication versus infection and their causes

Food-related illness is generally classified as either an intoxication, infection or toxin-mediated infection.

  • Intoxication: The term “food poisoning” generally refers to foodborne intoxication and illness brought about by the toxins present in food or water which are produced by certain bacteria on or in them, as well as the unhygienic handling and preparation of food. Foodborne intoxication may also be caused by toxins produced to the presence of heavy metals, chemicals and foreign substances in food, toxins absorbed from the environment by plants, animals or fish, or those that result from an animal or fish’s feeding habits. Toxin related food poisoning generally results in symptoms occurring soon after a contaminated food is ingested and usually result in rapid-onset gastro-intestinal distress (cramping and diarrhoea) and vomiting. (1)
  • Foodborne infection: Also referred to as foodborne illness is caused by the ingestion of live infectious pathogens (disease causing organisms) such as certain bacteria and all viruses or parasites which then grow and establish themselves in the gastrointestinal tract and lead to illness. Symptoms appear considerably later and last longer than those related to intoxication based food poisoning. Infections may also be transmitted more easily from one person to another.
  • Toxin-mediated infection: This type of foodborne illness occurs when certain live bacteria are ingested and invade the gastrointestinal tract. As they live and grow they the release illness inducing toxins, causing toxin-mediated infections. (2)

There are more than 200 known infectious contaminant causes / types of food poisoning and foodborne illness. Some of the most common causes are:

Bacteria that cause food poisoning

The most common cause of the majority of food poisoning instances can be linked to a bacterial contaminant. In many instances, the bacteria itself is not as harmful as the toxin it produces. Bacteria is commonly found in the intestines or digestive systems of healthy animals, and is transferred during the production process (i.e. when animals are slaughtered, and the intestines are used in meat products).

The various bacteria, their classifications and the names of the infections they cause (where applicable) are as follows:

  • E. coli (Escherichia coli) – induce a toxin-mediated infection: This bacterium is a common cause of diarrhoea in travellers and food poisoning and ingesting just a small amount can cause an infection. Contamination of food products occurs most often when E.coli bacteria present in the intestines of cattle come into contact with meat (commonly beef) during the slaughtering process. E. coli therefore typically spreads in undercooked ground beef products. It is also common in unpasteurised milk products, contaminated water or apple cider and may also spread through interpersonal contact when proper hygiene is not practiced. The onset of symptoms occurs between 1 and 8 days after infected food or beverages have been consumed and associated illness lasts 5 to 10 days (with most recovering in 6 to 8 days) if no complications arise. 
  • Salmonella – causes an infection known as Salmonellosis: Meat (raw), poultry, egg yolks and raw milk are common foodstuffs which salmonella can be traced back to, although it can rarely also contaminate fruit and vegetables. Any of these foods which are consumed raw or are inadequately cooked can easily cause food poisoning. Surfaces which come into contact with the bacterium, such as knives or food preparation boards, or even a person’s skin (when handling food) can spread this bacterium. It can take between 1 and 3 days for symptoms to appear following contact / ingestion with contaminated food and generally last 4 to 7 days. If an infection enters the bloodstream, symptoms can become life-threatening. 
  • Listeria monocytogenes – results in an infection known as Listeriosis: This bacterium is commonly found in unwashed raw produce (fruits and vegetables), processed foods such as luncheon meats and hot dogs, smoked seafood as well as unpasteurised milk and cheese products (particularly soft cheeses like brie, camembert and blue cheese). Water and soil can also become contaminated with Listeria and thus affect foods which come into contact with either of these. The onset of symptoms occurs between 9 and 48 hours following ingestion and may last a matter of days or weeks.
  • Campylobacter jejuni – results in an infection referred to as Campylobacteriosis: This bacterium is most often traced back to contaminated meat and poultry products which occurs when animal faeces comes into contact with meat surfaces during slaughter. When these foods are undercooked and consumed, food poisoning is often the result. Contaminated water, unpasteurised milk and shellfish are other common substances this bacterial contaminant can be traced back to. The onset of symptoms typically occurs within 2 to 5 days of ingestion and can last 2 to 10 days. 
  • Clostridium. botulinum (botulism) – causes intoxication: This is a very rare cause of food poisoning, however, when it does occur, it is potentially life-threatening and considered a medical and public health emergency. This bacterium grows on certain foods and produces dangerous toxins. This occurs most often in canned food items with low oxygen and low acidity (either home-canned or those produced commercially). Smoke or salted fish products, or potatoes that are baked in aluminium foil which are kept at warm temperatures for long periods of time can be easy targets for this bacterium contaminant as the bacterium is heat resistant. Onset of symptoms are most often noted within 12 to 72 hours following ingestion, although may show within 4 hours to 8 days and the duration of the associated illness varies based on the individual and whether any further complications arise as a result. (3)
  • Clostridium perfringens – causes a toxin-mediated infection: This infectious agent is often traced back to meat foodstuffs, including stews and gravies. The bacterium produces a spore (durable coats of protein organisms) which germinates in cooked meat, and easily spreads when food is chilled too slowly or are prepared in large quantities and warmed for extended periods before they are served (often at catered events, in cafeterias or institutions such as nursing homes, schools and prisons). (4) Onset of symptoms generally occurs between 6 and 24 hours after contaminated foods have been consumed. Illness as a result usually lasts for a day or less, although severe cases, symptoms may last up to two weeks.
  • Shigella – results in an infection referred to Shigellosis: Responsible for traveller’s diarrhoea in developing countries and often causing food poisoning in childcare and educational institutions, shigella can be transmitted via contaminated food and water. Produce which is raw and read-to-eat such as salads or sandwiches and seafood can easily become contaminated with this bacterium. (5) A person handling food can also easily transfer shigella and contaminate food. Onset of symptoms typically occurs within 24 to 72 hours after consuming contaminated foods and lasts between 2 and 7 days. 
  • Staphylococcus aureus – causes intoxication: This bacterium can cause food poisoning when a person preparing food contaminates it or the equipment and surfaces it is prepared on cause cross contamination and the food is subsequently not refrigerated properly. Staphylococcus (or staph) bacteria multiply rapidly at room temperature, producing a toxin that causes foodborne illness. Hand contact, sneezing or coughing can also easily transfer this contaminant, spreading the bacterium further. Pre-packaged salads, sandwiches, cream sauces or filled pastries and meats are some of the most common traceable foods for contamination with this bacterium. Food poisoning symptoms can take place within 1 to 6 hours following exposure and last a day or two.
  • Vibrio Infections: Two strains of vibrio exist, namely Vibrio vulnificus (V. vulnificus) and parahaemolyticus (V. parahaemolyticus). Raw or undercooked seafood, such as mussels, oysters and scallops, or even contaminated seawater can cause food poisoning from either of these bacteria. Onset of symptoms as a result of v. vulnificus can occur within 1 to 7 days and those related to a V. parahaemolyticus infection generally appear in 2 to 48 hours following ingestion of contaminated foodstuffs. The duration of illness associated with poisoning from these bacteria is 2 to 8 days.
  • Bacillus Cereus (or B.Cereus) – causes intoxication: This bacterium is known to produce toxins that cause food poisoning characterised by either nausea and vomiting (caused by emetic toxins) or diarrhoea. This bacterium occurs in raw or poorly cooked rice (originating from soil) and foods that have been left at room temperature for extended periods. The onset of symptoms for the diarrhoeal variant occur after 6 to 15 hours and the emetic type anything from 30 minutes to 6 hours after ingestion. Both types generally last for approximately a day.

Viruses that cause food poisoning

Viruses which can cause contamination, can in rare instances, be life-threatening, they are all classified as foodborne infections. Those which are capable of being transmitted through foodstuffs include:

  • Hepatitis A: This virus can easily contaminate shellfish or raw and ready-to-eat produce. Ingesting contaminated water is another means by which this virus can cause food-poisoning. Noticeable symptoms of contamination usually occur within 28 days, although this varies, and these may range from 15 to 50 days. The duration of the resultant illness varies from 2 weeks up to 3 months.
  • Norovirus (Norwalk virus): Foods this virus is commonly traced back to include shellfish (often oysters), and ready-to-eat / raw pre-packaged foods, which have often been handled in contaminated water. An infected food handler can also spread contamination. Onset of symptoms commonly takes place between 12 and 48 hours and the illness lasts between 1 and 3 days, however, young children, the elderly and for those with compromised immune systems requiring hospitalisation, it may take 4 to 6 days to recover. 
  • Rotavirus: This virus is the most common cause of food related illness in toddlers and young children and is rarely encountered in adulthood as immunity develops once one has been exposed to it (usually in childhood). Produce which is raw and packaged as ready-to-eat can easily become contaminated with this virus. An infected food handler can also easily spread the virus, causing symptoms between 1 and 3 days after consuming affected foodstuffs and last up to 9 days.

Parasites that cause food poisoning

A parasite which contaminates food can become fairly dangerous. A parasite is capable of living in the digestive tract for quite some time, sometimes undetected for a number of years. Those who have compromised immune systems or fall pregnant are at increased risk of severe side-effects due to food poisoning from parasite contaminants. All instances of parasite-induced foodborne illnesses are regarded as infections.

  • Anisakis simplex: This parasite, also known as herring worm infect crustaceans, fish, squid and marine mammals as they head up the food chain. Infection in humans occurs when infected raw or undercooked fish or squid are ingested. Symptoms appear anything from 2 hours to 2 weeks after the consumption of infected seafood and may persist for months. Treatment with antiparasitic medication helps to eradicate the parasite.
  • Cryptosporidium homonis and Cryptosporidium parvum: Also known as “crypto”, infection with these species, referred to as cryptosporidiosis usually occurs after exposure to contaminated drinking or recreational water and occasionally through the ingestion of contaminated food. Symptoms, when present, range in severity, appearing between 2 and 10 days after infection and lasting anything from a few days to over a month. The primary symptom is watery diarrhoea and may be accompanied by stomach cramps, nausea, vomiting, fever, dehydration and weight loss. Those with healthy immune systems will not generally require treatment. When treatment is required, antidiarrhoeal medication such as nitazoxanide may be prescribed.
  • Cyclospora cayetanensis: This parasite causes a condition known as cyclospora, causing explosive, watery diarrhoea after ingesting contaminated food or water. The onset of symptoms occurs approximately 1 week after ingestion and if not treated, can last up to a month or more. Trimethoprim/sulfamethoxazole antibiotics are prescribed to treat cyclospora.
  • Entamoeba histolytica: This parasitic infection spreads when food or water that has come into contact with contaminated faeces is ingested. This causes a disease known as amoebiasis (also spelled amebiasis). While most infections are asymptomatic (i.e. do not cause symptoms) in an infected person, dysentery and disease outside of the intestines may occur. Amoebic (amebic) liver abscess and amoebic colitis are common complications caused by infestation although organs other than the liver and colon may also be affected. Medication is used to rid the body of this parasite but if it is not entirely effective, reinfestation may occur.
  • Giardia / Giardia duodenalis / Giardia lamblia / Giardia intestinalis: This parasite infects the small intestine and causes a diarrhoeal illness known as giardidiasis. It is most often traced to raw or ready-to-eat produce, such as pre-packaged salads or fruits and vegetables. Contaminated water, soil, surfaces or an infected food handler can also easily spread contamination. Symptoms most often present themselves within 1 to 2 weeks and in generally healthy individuals last 2 to 6 weeks.
  • Toxoplasma gondii: This parasite can exist in a person’s body without causing symptoms or making them sick. However, in some it does cause illness and when this occurs it is referred to as toxoplasmosis. Meat that is inadequately cooked or comes into contact with animal faeces (specifically that of a cat) most often leads to this underlying cause of food poisoning. Soil or water contamination can also transfer the parasite, spreading infection. Pregnant women are most susceptible to poisoning as unborn babies can contract it if the initial infection takes place during the gestation period. The onset of symptoms typically occurs within one to three weeks of ingesting contaminated food. Toxoplasmosis infections generally last between 2 and 4 weeks.
  • Trichinella spiralis: This species of Trichinella, commonly known as round worm, is found across the globe in many omnivorous and carnivorous animals. Trichinellosis (also spelled trichinosis) occurs when the trichinella spiralis parasite is acquired by consuming improperly processed, raw or undercooked meat (usually pork or wild game) or meat products contaminated with the parasite. Gastrointestinal symptoms (stomach cramps, diarrhoea, nausea and/or vomiting) usually occur within 1 to 2 days of consumption of infected meat. The classic symptoms of infection follow, usually 2 weeks later and may last up to 8 weeks. Symptoms range in severity and some may even be asymptomatic. A doctor will prescribe medication based on the symptoms exhibited and lab test results.

Chemicals / toxins that cause food poisoning

Poisoning can also occur via contamination with certain chemicals which come into contact with various foods. Examples of chemicals which cause food poisoning due to intoxication include:

  • Scrombroid poisoning (scombrotoxic histamine): Fish and fish products, such as anchovies, herrings, mackerel, salmon, yellowfin tuna and sardines, which are inadequately cooked or poorly stored (stimulating bacteria / toxin growth) can lead to poisoning within 1 to 2 hours following ingestion. Histamine and other similar chemicals cause the toxin to occur, thus poisoning those who consume contaminated food sources. A contaminated fish may have a metallic or peppery taste when being eaten. Cooking can help to reduce risk of falling ill, but the toxin can remain in the fleshy tissues of fish products and still be absorbed by a person consuming it. Symptoms occur rapidly, usually between 15 minutes and 2 hours and may be similar to those of a food allergy and include flushing, swallowing difficulty and even shortness of breath. Symptoms often resolve within 10 to 14 hours, sometimes up to 2 days (although this is rare). 
  • Ciguatera poisoning: Reef fish are most commonly affected by this toxin which is produced by dinoflagellates (marine plankton) that live in tropical and subtropical waters. Fish commonly affected include barracuda, snapper or grouper. Symptoms of contamination include diarrhoea, vomiting, muscle aches, headache, ataxia (balance problems), as well as numbness and tingling sensations. Onset of symptoms can occur within 6 to 24 hours of consuming a contaminated fish source and poisoning generally resolves within 1 to 4 days. (6)


1. NC Health and Public Services. 3 March 2017. Food Poisoning & Foodborne Illnesses:  [Accessed 30.10.2017]

2. San Francisco City and County Department of Public Health . How foodborne illness starts:  [Accessed 30.10.2017]

3. World Health Organisation. October 2017. Botulism (fact sheet):  [Accessed 30.10.2017]

4. Centers for Disease Control and Prevention. 10 January 2017. Foodborne Germs and Illnesses:  [Accessed 30.10.2017]

5. Food 6 November 2017. Shigella  [Accessed 30.10.2017]

6. PMC U.S National Library of Medicine National Institutes of Health. September 2008. Ciguatera Fish Poisoning: Treatment, Prevention and Management   [Accessed 30.10.2017]

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