Food poisoning is caused by eating food contaminated with bacteria, viruses, parasites, or natural toxins. In the United States alone, these contaminants cause an estimated 9.9 million foodborne illnesses every year from just seven major pathogens, leading to 53,300 hospitalizations and 931 deaths. The specific cause depends on what you ate, how it was handled, and how long it sat at unsafe temperatures.
Bacteria: The Most Common Culprits
Bacteria are responsible for the majority of food poisoning cases, and they contaminate food in different ways. Some are already present in raw animal products. Others multiply when cooked food sits out too long. Here are the ones most likely to make you sick.
Norovirus is technically a virus, but it tops the list overall with 5.5 million estimated illnesses per year in the U.S. It spreads easily through contaminated food, water, and surfaces, and a sick food handler is often the source.
Campylobacter causes roughly 1.87 million illnesses annually, most often from undercooked chicken, raw (unpasteurized) milk, contaminated water, and shellfish. It’s one of the leading bacterial causes of diarrheal illness.
Salmonella accounts for about 1.28 million illnesses and causes more deaths than any other foodborne pathogen on the CDC’s tracking list: 238 per year. It shows up in an unusually wide range of foods, including chicken, pork, eggs, fruits, vegetables, nuts, and sprouts. You can also pick it up from contact with reptiles, amphibians, and baby chicks.
Clostridium perfringens causes around 889,000 illnesses a year and is sometimes called the “buffet germ.” It thrives in large batches of food, especially beef, poultry, and gravies, that are left sitting at warm temperatures for extended periods. Steam tables and catered events are common settings for outbreaks.
E. coli (specifically the toxin-producing strains like O157:H7) causes about 357,000 illnesses annually. The classic sources are undercooked ground beef, raw milk, soft cheeses made from raw milk, and raw produce like lettuce, leafy greens, and sprouts. E. coli is particularly dangerous for young children: kidney failure strikes 1 out of 7 children under age 5 diagnosed with O157 infection.
Staphylococcus aureus (staph) is different from most foodborne bacteria because the toxin it produces, not the bacteria itself, makes you sick. Staph lives on human skin, and people transfer it to food when they don’t wash their hands. Foods that aren’t cooked after handling are especially risky: sliced deli meats, puddings, pastries, and sandwiches.
Listeria causes only about 1,250 illnesses per year, but it’s disproportionately deadly. Of those cases, 1,070 lead to hospitalization and 172 to death. It grows in unpasteurized dairy products, deli meats, hot dogs, refrigerated smoked seafood, and soft cheeses. Pregnant women are 10 times more likely to get a Listeria infection than the general population, and people on dialysis are 50 times more likely.
Viruses and Parasites
Beyond norovirus, hepatitis A is the other major viral cause of foodborne illness. It typically spreads through raw or undercooked seafood and contaminated raw produce, and it can cause long-lasting liver disease, making it far more serious than the typical stomach bug.
Parasites enter the food supply through different routes. Tapeworms can infect people through undercooked meat or direct contact with animals. Fish-borne parasites spread exclusively through contaminated seafood. Others, including Cryptosporidium and Giardia, get into the food chain through contaminated water or soil and end up on fresh produce. The parasite Toxoplasma causes enough severe illness to account for 848 hospitalizations and 44 deaths per year in the U.S.
Natural Toxins in Food
Not all food poisoning comes from living organisms. Some comes from toxins that are naturally present in food or produced by molds.
Ciguatera fish poisoning occurs when you eat reef fish that have accumulated toxins from tiny marine organisms called dinoflagellates. Barracuda, black grouper, dog snapper, and king mackerel are among the fish known to carry these toxins. Symptoms include nausea, vomiting, and a distinctive neurological effect: tingling in the fingers and toes. There’s no specific treatment, and symptoms can linger for weeks or months.
Mycotoxins are toxic compounds produced by certain molds that grow on cereals, dried fruits, nuts, and spices. What makes them particularly problematic is that they’re chemically stable and survive food processing, so they can persist in finished products even after cooking.
How Temperature Creates Risk
Bacteria grow most rapidly between 40°F and 140°F, a range the USDA calls the “Danger Zone.” Within that range, bacteria can double in number in as little as 20 minutes. This is why food left sitting on a counter, at a picnic, or on a buffet becomes increasingly risky over time.
The basic rules are straightforward: keep hot food at or above 140°F, keep cold food at or below 40°F, and never leave food out of refrigeration for more than 2 hours. If the ambient temperature is above 90°F (a summer barbecue, for example), that window shrinks to 1 hour. When reheating leftovers, bring them to an internal temperature of 165°F.
One bacterium that exploits this window particularly well is Bacillus cereus, which thrives in cooked rice, sauces, soups, and other prepared foods that sit at room temperature too long. This is why leftover rice is a surprisingly common cause of food poisoning.
Cross-Contamination During Prep
Many food poisoning cases don’t start with a contaminated ingredient. They start with contaminated hands, cutting boards, or countertops transferring germs from raw meat to something that won’t be cooked.
Raw meat, poultry, seafood, and eggs can spread germs to ready-to-eat food at every stage: during grocery shopping (if juices leak onto other items), in the refrigerator (if raw meat drips onto lower shelves), and during preparation. Use separate cutting boards for raw meat and for produce or bread. Wash utensils, cutting boards, and countertops with hot, soapy water after preparing each food item.
One counterintuitive point: washing raw chicken before cooking actually increases your risk. Rinsing it splashes bacteria onto the sink, counter, and nearby surfaces. The chicken doesn’t need washing because cooking it to the proper internal temperature kills the bacteria.
Foods You Might Not Suspect
Some high-risk foods are obvious, like undercooked chicken or raw oysters. Others catch people off guard.
Raw flour is one of them. It doesn’t look like a raw food, but it hasn’t been treated to kill pathogens. Harmful bacteria like E. coli and Salmonella can contaminate grain in the field, and grinding or bleaching flour doesn’t kill them. Those germs are only destroyed when flour is baked or cooked, which is why eating raw cookie dough or cake batter carries real risk.
Sprouts (alfalfa, bean, clover) are another frequent source of outbreaks. The warm, humid conditions needed to grow sprouts are also ideal for bacterial growth, and sprouts are typically eaten raw. Honey is safe for adults but can cause infant botulism in babies under one year old, whose digestive systems can’t yet handle the Clostridium botulinum spores it may contain. Improperly home-canned foods, especially low-acid vegetables, are a classic source of botulism in adults.
Why Symptoms Appear at Different Times
One reason food poisoning is hard to trace is that different pathogens have very different incubation periods. Staph food poisoning hits fast, within 1 to 6 hours, because the toxin is already in the food when you eat it. Salmonella typically takes 6 to 48 hours. E. coli can take anywhere from 1 to 8 days, which means the meal that made you sick might have been nearly a week ago, not the one you ate last night.
This variation also explains why two people can eat the same meal and get sick at different times, or why only one person at a dinner gets ill. Individual immune function, the amount of contaminated food consumed, and the specific pathogen all play a role.
Who Gets Hit Hardest
Anyone can get food poisoning, but four groups face significantly higher risks of severe illness. Adults 65 and older have immune systems and organs that don’t recognize and clear harmful germs as efficiently. Nearly half of people in this age group with confirmed Salmonella, Campylobacter, Listeria, or E. coli infections end up hospitalized.
Children under 5 have immune systems still in development, and food poisoning can become dangerous quickly because diarrhea leads to dehydration faster in small bodies. Children in this age group are three times more likely to be hospitalized from a Salmonella infection.
People with weakened immune systems, whether from diabetes, liver or kidney disease, HIV, autoimmune disorders, or cancer treatment, are less equipped to fight off foodborne pathogens. Pregnant women face elevated risk as well, particularly from Listeria, which can cause miscarriage, stillbirth, or serious illness in newborns.