Food poisoning, also called foodborne illness, occurs when a person consumes food or drink contaminated with a harmful biological agent, toxin, chemical contaminant or, in some circumstances, another hazardous substance. Infectious causes include bacteria, viruses and parasites, while some illnesses result from toxins already present in food rather than from organisms multiplying inside the person. Important foodborne pathogens include Salmonella, Campylobacter, pathogenic E. coli, Listeria, norovirus, hepatitis A virus and various toxin-producing bacteria. The term therefore describes a route of exposure and resulting illness rather than one single disease. Different causes have different incubation periods, symptoms, complications and treatments.
Bacteria cause food poisoning through several distinct mechanisms. Some bacteria multiply in food and produce toxins before the food is eaten, so symptoms can begin relatively rapidly after consumption; Staphylococcus aureus is a classic example of toxin-mediated food poisoning. Other bacteria, including Salmonella and Campylobacter, are swallowed alive and subsequently infect the gastrointestinal tract. Some pathogenic E. coli strains produce toxins that can damage intestinal or other tissues. Clostridium perfringens can multiply in inadequately stored food and cause diarrhoea and abdominal cramps, while Clostridium botulinum produces botulinum neurotoxin, which affects nerve function and can cause potentially life-threatening paralysis.
Viruses are another major cause of foodborne illness. Norovirus is especially important because it can contaminate food when an infected person handles it, but it can also spread through contaminated water, surfaces and direct contact. Hepatitis A virus can similarly be transmitted through contaminated food or water. Viral gastroenteritis acquired through food may therefore resemble gastroenteritis acquired through other routes. The presence of a virus in food does not necessarily mean the food was intrinsically unsafe when produced; contamination can occur during harvesting, processing, preparation, serving or handling. Norovirus can also contaminate food through tiny particles of vomit or faeces and can spread readily between people.
Parasites can cause foodborne illness when contaminated food or water introduces organisms such as Giardia, Cryptosporidium, Cyclospora or certain tapeworms into the body. Their incubation periods can be considerably longer than those of some bacterial toxins, and illness may involve persistent diarrhoea, abdominal discomfort, bloating, fatigue or weight loss. Some parasites have particular associations with undercooked meat or fish, while others are transmitted through contaminated water or produce. The distinction between infection and intoxication is useful here: infection generally involves the organism establishing itself in the host, whereas intoxication can occur because a preformed toxin is consumed.
Contamination can happen at virtually any stage of the food chain. Produce can become contaminated through soil, irrigation water, animal faeces or handling; meat can acquire organisms during slaughter and processing; seafood can become contaminated through polluted water; and prepared foods can become contaminated by infected food handlers or contaminated utensils and surfaces. Food can also become unsafe after cooking if it is subsequently exposed to pathogens. Cross-contamination occurs when microorganisms are transferred from one food, utensil, surface or pair of hands to another, particularly from raw meat, poultry, seafood or eggs to foods that will not subsequently be cooked.
Temperature control is another major factor. Bacteria can multiply rapidly when perishable food is held in conditions that allow substantial growth, whereas adequate cooking can destroy many vegetative bacterial cells and prompt refrigeration slows the growth of many organisms. Food should therefore be cooked sufficiently and refrigerated promptly rather than left for prolonged periods at temperatures favourable to microbial multiplication. Freezing generally prevents or greatly slows microbial growth but does not reliably eliminate every pathogen. Some toxins and spores are also unusually resistant to heat, which means that simply reheating food is not always sufficient to make previously mishandled food safe.
Certain foods carry greater inherent or practical risk because they are commonly eaten raw, lightly cooked, minimally processed or easily contaminated. Examples include raw or undercooked poultry, meat, seafood and eggs, unpasteurised milk and juice, raw sprouts, unwashed produce and raw dough or batter containing uncooked flour. Raw oysters and other filter-feeding shellfish are particularly relevant to norovirus and other foodborne pathogens because they can concentrate contaminants from surrounding water. Risk is also influenced by preparation, storage, source and the susceptibility of the person consuming the food.
Not every episode of vomiting or diarrhoea after eating is necessarily food poisoning. Symptoms may instead result from norovirus transmitted directly from another person, a pre-existing gastrointestinal disorder, food intolerance, medication, alcohol or another illness. Establishing the exact food responsible can also be difficult because incubation periods differ and people commonly consume multiple foods before becoming ill. Some foodborne illnesses can produce serious complications, including kidney damage, haemolytic uraemic syndrome, neurological disease or invasive infection. Young children, adults aged 65 and over, pregnant people and people with weakened immune systems are among those at increased risk of severe foodborne illness.
