Most foodborne illness outbreaks are caused by contaminated food, especially when germs are introduced before final preparation and when sick food workers handle ready‑to‑eat foods with their bare or gloved hands. Viral agents like norovirus and bacterial contamination from animal or environmental sources are the leading drivers behind these outbreaks.

What causes most outbreaks?

Public health surveillance in the United States shows that the majority of documented contributing factors fall into two big categories.

  • Contamination before food reaches the kitchen (on farms, in processing plants, or during transport) is reported as the single most common factor overall.
  • Contamination by infected food workers (especially through direct hand contact with ready‑to‑eat food) is consistently one of the top factors, particularly in viral outbreaks.

Role of germs and pathogens

Studies of restaurant‑associated outbreaks find that norovirus alone accounts for around 60% of outbreaks with a known cause, with Salmonella and Clostridium perfringens making up much of the rest.

These germs spread easily when hygiene is poor, food is kept at unsafe temperatures, or cooking and reheating do not reach times and temperatures needed to kill pathogens.

Key risk practices

Across recent national data, the most frequently documented risky practices include:

  • Handling food by an infected worker or carrier of a pathogen (bare‑hand or gloved‑hand contact).
  • Food held too long at room or warm temperatures during prep, service, or display, allowing germs to grow.
  • Inadequate cooking or reheating that fails to fully kill harmful bacteria or viruses.

Foods most often involved

Health departments report outbreaks linked to many foods, but some categories appear more often, including:

  • Raw or undercooked meat and poultry, seafood, and eggs.
  • Unpasteurized (raw) milk and dairy, and sometimes fresh produce contaminated before or during handling.

Quick safety takeaways

To reduce the risk of the kinds of factors that cause most foodborne illness outbreaks:

  1. Keep sick food workers away from food until fully recovered.
  2. Wash hands thoroughly and avoid direct contact with ready‑to‑eat foods.
  3. Cook and reheat foods to safe internal temperatures and keep hot foods hot and cold foods cold.

Information gathered from public forums or data available on the internet and portrayed here.