Contributing Factors of Foodborne Illness Outbreaks — National Outbreak Reporting System, United States, 2014–2022
Authors: Meghan M. Holst, Beth C. Wittry, Carolyn Crisp, Jeffrey Torres, D.J. Irving, David Nicholas
Journal: MMWR Surveillance Summaries (2025)

Approximately 800 foodborne illness outbreaks occur in the United States each year. These outbreaks include approximately 15,000 illnesses, 800 hospitalizations, and 20 deaths. Although illnesses from outbreaks account for a small portion of all foodborne illnesses, outbreak investigations reveal how these illnesses originate by offering crucial data through epidemiologic, environmental health, and laboratory analyses and aid in outbreak mitigation and prevention. 2014-2022. The Foodborne Disease Outbreak Surveillance System (FDOSS), via the National Outbreak Reporting System (NORS), captures data from foodborne enteric illness outbreak investigations in the United States. Epidemiology or communicable disease control and environmental health programs of state and local health departments collect and voluntarily report the data to NORS, which is managed by CDC. These data include information about cases (e.g., case counts, symptoms, duration of illness, and health care-seeking behaviors), laboratory specimens, settings of exposure, implicated food items, and contributing factors (i.e., how the outbreak occurred). A foodborne illness outbreak is defined as two or more cases of a similar illness associated with a common exposure (e.g., shared food, venue, or experience). Data collected from an outbreak investigation help the investigator identify contributing factors to the outbreak. Contributing factors are food preparation practices, behaviors, and environmental conditions that lead to pathogens getting into food, growing in food, or surviving in food and are grouped into three categories: contamination (when pathogens and other hazards get into food), proliferation (when pathogens that are already present in food grow), and survival (when pathogens survive a process intended to kill or reduce them). A total of 2,677 (40.5%) foodborne illness outbreaks reported during 2014-2022 with information on contributing factors were included in this analysis. Foodborne outbreak periods were categorized into three time frames: 2014-2016 (first), 2017-2019 (second), and 2020-2022 (third). Of the 2,677 outbreaks, 1,142 (42.7%) occurred during the first time frame, 1,130 outbreaks (42.2%) during the second time frame, and 405 outbreaks (15.1%) during the third time frame. The proportion of bacterial outbreaks increased from the first (41.9%) to the third time frame (48.4%), and the proportion of viral outbreaks decreased (33.3% to 23.2%). Over the three time frames, the proportion of outbreaks with a contamination contributing factor decreased (85.6%, 83.6%, and 81.0%, respectively). The proportion of outbreaks with a proliferation contributing factor category decreased from the first (40.3%) to the second time frame (35.0%), then increased during the third time frame (35.1%), and the proportion of outbreaks with a survival contributing factor category decreased from the first (25.7%) to the second time frame (21.9%), then increased during the third time frame (25.7%). The proportion of outbreaks with aquatic animals as an implicated food item increased from the first (12.0%) to the second time frame (18.5%), then decreased during the third time frame (18.3%). The proportion of outbreaks with land animals as an implicated food item decreased from the first (16.7%) to the second time frame (14.2%), then increased during the third time frame (15.1%).For outbreaks with a contamination contributing factor, the proportion of food contaminated by an animal or environmental source before arriving at the point of final preparation increased over the three time frames (22.2%, 27.7%, and 32.3%, respectively), and the proportion of outbreaks with contamination from an infectious food worker through barehand contact with food decreased (20.5%, 15.2%, and 8.9%, respectively). For the proliferation category, the proportions of outbreaks associated with allowing foods to remain out of temperature control for a prolonged period during preparation and during food service or display decreased over the three time frames (15.2%, 12.2%, and 9.9%, respectively; and 13.6%, 10.4%, and 8.9%, respectively), and the proportion of improper cooling of food decreased from the first (9.4%) to the second time frame (8.8%), then increased during the third time frame (10.9%). For the survival category, the proportion of outbreaks associated with inadequate time and temperature control during initial cooking/thermal processing of food decreased from the first (12.1%) to the second time frame (9.6%) and increased during the third time frame (12.1%).For bacterial outbreaks, cross-contamination of foods was among the top five contributing factors during the first (22.0%) and second time frames (20.8%) but not during the third time frame. Inadequate time and temperature control during initial cooking of food was among the top five contributing factors during all three time frames (23.8%, 20.4% and 20.9%, respectively). Improper cooling was not among the top five contributing factors during the first and second time frames but was during the third time frame (17.3%). For viral outbreaks, contamination from an infectious food worker through barehand contact with food was among the most common contributing factors during the first (47.1%) and second time frames (37.7%) and decreased to the third most common contributing factor during the third time frame (28.7%). Contamination from an infectious food worker through gloved-hand contact with food was among the top five contributing factors during the first (32.1%) and second time frame (25.5%) and was the most common contributing factor during the third time frame (42.5%). Many foodborne illness outbreaks occur because of contamination of food by an animal or environmental source before arriving at the point of final preparation. Most viral outbreaks are caused by contamination from ill food workers. The decrease in the proportion of viral outbreaks and the proportion of outbreaks with a contamination contributing factor during 2020-2022 might be attributed to effects from the COVID-19 pandemic. Nonpharmaceutical interventions (e.g., increased glove use, cleaning and disinfection, and closure of restaurant dining areas) implemented during the COVID-19 pandemic likely led to a reduction in norovirus, which is typically spread by infectious food workers. Two common contributing factors to bacterial outbreaks are allowing foods to remain out of temperature control for a prolonged period and inadequate time and temperature control during cooking. Proper time and temperature controls are needed to effectively eliminate bacterial pathogens from contaminated foods and ensure safe food operations. Retail food establishments can follow science-based food safety guidelines such as the Food and Drug Administration Food Code and Hazard Analysis and Critical Control Points (HACCP) plans. Restaurant managers can mitigate contamination by ill food workers by implementing written policies concerning ill worker management, developing contingency plans for staffing during worker exclusions, and addressing reasons why employees work while sick. Health department staff members who investigate outbreaks and conduct routine inspections can encourage restaurants to follow their HACCP plans and other verified food safety practices, such as cooling, to prevent outbreaks.