Why Foodborne Outbreaks Happen: Contributing Factors, United States, 2014–2022

The problem

About 800 foodborne illness outbreaks happen in the United States each year, with about 15,000 illnesses, 800 hospitalizations and 20 deaths. Outbreak illnesses are a small share of all foodborne illness, but investigating them shows how illnesses start — and how to prevent them.

How the data are collected

State and local health departments investigate outbreaks and voluntarily report them to CDC's Foodborne Disease Outbreak Surveillance System (FDOSS) through the National Outbreak Reporting System (NORS): cases, lab results, where food was prepared and eaten, the foods involved, and contributing factors — how the outbreak happened.

• An outbreak is two or more cases of a similar illness linked to a common exposure, such as a shared food or event.
• Contributing factors are practices, behaviors and conditions that let pathogens get into, grow in or survive in food, in three groups:
• • contamination — pathogens or other hazards get into food;
  • proliferation — pathogens already there grow;
  • survival — pathogens survive a step meant to kill or reduce them.

CDC had not published a summary of contributing factors since 2016. This one looks at changes over time, the COVID-19 pandemic, and differences between bacterial and viral outbreaks.

The outbreaks analyzed

Of 6,618 outbreaks reported in 2014–2022, 3,788 (57.2%) had no contributing factor and were left out, along with a few with conflicting pathogen categories or implausible factors (for example, 126 viral outbreaks reported with growth or survival factors — viruses don't multiply outside the body). That left 2,677.

Institutions include schools, prisons, offices, hospitals and nursing homes.

The most common factors

• Food contaminated before final preparation, by an animal or environmental source: the most common factor overall (26.0%), and rising — 22.2%, 27.7%, 32.3%.
• A sick food worker's bare hands: second overall (16.5%), but falling — 20.5%, 15.2%, 8.9%.
• A sick worker's unknown or indirect contact with food: 13.1%.
• Food left out of temperature control too long:
• • during preparation: 13.1%, falling 15.2%, 12.2%, 9.9%;
  • during service or display: 11.5%, falling 13.6%, 10.4%, 8.9%.
• Inadequate cooking time and temperature: 11.0% (12.1%, 9.6%, 12.1%); inadequate reheating: 7.3%.
• Improper cooling: 9.4%, 8.8%, 10.9%.

Bacterial versus viral

Bacterial outbreaks — top factors:

• contamination before final preparation: the most common in every period (41.8%, 42.5%, 50.5%);
• inadequate cooking: top five throughout (23.8%, 20.4%, 20.9%);
• food left out too long during preparation: top five throughout, but falling (30.1%, 22.7%, 15.3%);
• cross-contamination: top five in the first two periods (22.0%, 20.8%), not in the third;
• improper cooling: entered the top five only in 2020–2022 (17.3%).

Viral outbreaks (mostly spread by sick workers):

• bare-hand contact by a sick worker: the top factor in the first two periods (47.1%, 37.7%), third in 2020–2022 (28.7%);
• gloved-hand contact by a sick worker: top five at first (32.1%, 25.5%), then the most common (42.6%);
• a sick non-food-worker's contact with food: 9.7% in the first period, and fourth in the third (11.7%).

What explains the changes

• COVID-19: outbreaks reported fell 64.5% from the first period to the third — likely because health departments lacked staff to investigate, and because people ate at home more, restaurants closed, and large events such as weddings were discouraged.
• Fewer viral outbreaks: pandemic measures — more gloves, more cleaning and disinfection, closed dining rooms — likely cut norovirus, usually spread by sick food workers. The same cleaning likely reduced cross-contamination.
• More institutional outbreaks: schools, prisons and nursing homes kept serving meals, often cooked in bulk and held longer to deliver to rooms or stagger mealtimes — which could explain more temperature problems. Staff turnover and absences also left new or overstretched workers.
• Better at spotting poor cooling: a decade of research on cooling may have made investigators better at recognizing it.
• Home cooking counts too: about 16.5% of outbreaks in each period involved food made in private settings — homes, picnics, potlucks and celebrations. Plain-language advice is at FoodSafety.gov.

What to do

• Restaurants and retailers: follow science-based guidance such as the FDA Food Code and Hazard Analysis and Critical Control Points (HACCP) plans; cook foods properly (raw chicken to 165°F [73.9°C]); and control time and temperature, including cooling.
• Managers: write policies on sick workers, plan staffing for when they're excluded, and deal with why employees work while ill — not wanting to leave coworkers short, or being unable to lose pay. When short-staffed, simplify the menu so steps like cooling and handwashing aren't skipped.
• Health departments: encourage restaurants to follow their HACCP plans and verified practices; check cooling logs, thermometers and practices; and identify contributing factors whenever possible.
• Governments: consider pandemic preparedness plans for safe food operations, from farm to retail.

Limitations

NORS is voluntary, so it may not capture every outbreak; reporting varies with health departments' training and staffing; states can update reports at any time; and the contributing-factor definitions were revised in 2022, so some may be misclassified.

Sources

Based on "Contributing Factors of Foodborne Illness Outbreaks — National Outbreak Reporting System, United States, 2014–2022," by Meghan M. Holst, Beth C. Wittry, Carolyn Crisp, Jeffrey Torres, D.J. Irving and David Nicholas, MMWR Surveillance Summaries, Centers for Disease Control and Prevention; in the public domain, as MMWR declares for everything it publishes. Where the report's abstract and results differ — gloved-hand contact in 2020–2022 (42.5% or 42.6%) and the proliferation trend — this page follows the results.