Biological hazards are the most common type of food hazard, and it’s not close. Of the three categories food safety experts track (biological, chemical, and physical), living organisms like bacteria and viruses cause the vast majority of foodborne illness. Globally, unsafe food causes 600 million cases of foodborne disease every year, and the single biggest contributor in the United States is norovirus, responsible for roughly 5.5 million foodborne illnesses annually.
Three Categories of Food Hazards
Food hazards fall into three groups: biological, chemical, and physical. Biological hazards include bacteria, viruses, and parasites. Chemical hazards cover pesticide residues, cleaning agents, heavy metals like lead and mercury, and industrial contaminants. Physical hazards are foreign objects in food, such as glass fragments, metal shavings, or bone chips.
All three matter, but they differ dramatically in how often they make people sick. Chemical and physical hazards tend to affect smaller numbers of people in isolated incidents. Biological hazards sicken millions every year because microorganisms multiply rapidly in food and spread easily from person to person and surface to surface. One in ten people worldwide falls ill from contaminated food each year, and the overwhelming majority of those cases trace back to a virus or bacterium.
Norovirus: The Leading Cause of Foodborne Illness
Norovirus is the single most common foodborne pathogen in the United States. CDC estimates from 2019 attribute approximately 5.5 million domestically acquired foodborne illnesses to norovirus, along with 22,400 hospitalizations and 174 deaths. That’s more than four times the number of illnesses caused by the next most common pathogen, Campylobacter.
The virus spreads with alarming efficiency. An infected person sheds billions of norovirus particles, and it takes only a tiny amount to contaminate food and make someone sick. The most common route is an infected food worker touching ready-to-eat items like salads, fresh fruit, or sandwiches with contaminated hands. Food can also pick up the virus when it’s placed on a contaminated surface, or when microscopic droplets from vomit land on nearby food or countertops.
Certain foods show up repeatedly in norovirus outbreaks: leafy greens like lettuce, fresh fruits, and shellfish (especially oysters). Oysters are a particular risk because they filter large volumes of water. If that water is contaminated with sewage, the virus concentrates inside the shellfish. Fruits and vegetables can be contaminated in the field if they’re irrigated or sprayed with contaminated water.
Salmonella: Less Common but More Deadly
Salmonella causes far fewer total illnesses than norovirus, around 1.28 million per year in the U.S., but it’s considerably more dangerous when it strikes. Salmonella leads to 12,500 hospitalizations and 238 deaths annually, making it the top cause of death among major foodborne pathogens. The infections tend to be more severe, with symptoms like high fever, bloody diarrhea, and dehydration that can require medical intervention, especially in young children and older adults.
Other significant biological hazards include Campylobacter (often linked to undercooked poultry), Clostridium perfringens (which thrives in large batches of food left at unsafe temperatures), E. coli, and Listeria. Listeria is relatively rare but has a high fatality rate, particularly for pregnant women and people with weakened immune systems. Together, seven major pathogens account for an estimated 9.9 million foodborne illnesses in the U.S. each year.
Chemical and Physical Hazards
Chemical hazards in food come from two main sources: substances added during production (pesticides, cleaning agents) and environmental contaminants that accumulate over time. Heavy metals like arsenic, lead, mercury, and cadmium enter food through contaminated soil, water, or air. These are especially concerning for developing brains, from the womb through early childhood. Industrial chemicals, including certain nonstick compounds (PFAS) and dioxins, can also build up in the food supply. Pesticide residues on produce are regulated, but trace amounts often remain on crops.
Physical hazards are foreign objects that end up in food during harvesting, processing, or preparation. Glass, metal fragments, pieces of bone, and plastic are the most common culprits. The FDA considers hard or sharp objects between 7 and 25 millimeters long a serious risk for cutting or perforating tissues in the mouth, throat, or digestive tract. Objects smaller than 7 mm rarely cause injury in healthy adults, though they can still be dangerous for infants, elderly people, and surgery patients.
While chemical and physical hazards can cause real harm, they affect far fewer people than biological hazards do in any given year.
Why Biological Hazards Spread So Easily
Bacteria multiply fastest between 40°F and 140°F, a range food safety experts call the “danger zone.” In that temperature window, bacterial populations can double every 20 minutes. A single piece of chicken left on a counter for two hours can go from a safe bacterial load to a dangerous one. When the ambient temperature is above 90°F, that window shrinks to just one hour.
This is why temperature control is the backbone of food safety. The minimum safe internal temperatures for the foods most commonly linked to biological hazards are: 165°F for all poultry, 160°F for ground meats, and 145°F for fish and shellfish. These temperatures are high enough to kill the bacteria and parasites most likely to be present in each type of food.
Foods Most Likely to Carry Hazards
Some foods are inherently riskier than others. Poultry and ground meats are frequent carriers of Salmonella and Campylobacter because the bacteria naturally live in animal intestines and can spread during processing. Ground meat is higher risk than whole cuts because grinding distributes any surface bacteria throughout the product.
Leafy greens and fresh fruits are common vehicles for norovirus and E. coli because they’re usually eaten raw, giving pathogens no chance to be killed by heat. Shellfish, particularly oysters eaten raw, concentrate whatever pathogens are present in the surrounding water. Unpasteurized dairy and raw sprouts round out the list of higher-risk foods because both provide ideal conditions for bacterial growth.
Practical Ways to Reduce Your Risk
Most foodborne illness at home comes down to four failures: not washing hands thoroughly, not cooking food to the right temperature, leaving perishable food in the danger zone too long, and cross-contaminating ready-to-eat foods with raw meat juices. Using a food thermometer is the only reliable way to confirm that meat, poultry, or seafood has reached a safe internal temperature. Color and texture are not accurate indicators.
For produce, washing under running water removes some surface contamination, though it won’t eliminate all pathogens. Refrigerating perishable foods promptly, keeping raw meats separated from other ingredients, and being cautious with buffet-style meals that sit out for extended periods all reduce risk. If you’re handling food while recovering from a stomach illness, you can still shed norovirus for days after symptoms stop, so avoiding food preparation during that window protects the people around you.