A cluster of gastrointestinal illnesses is traced to a widely distributed brand of prewashed salad mix that was contaminated at a single processing plant. Which of the seven contributing factors best explains how this outbreak expanded rapidly across multiple states?

• A. Population increase: because urban populations are larger now, people ate more salad, which raised per-capita susceptibility sufficient to explain an outbreak in multiple states even if the product was only locally distributed.
• B. Microbe evolution: the bacterium underwent rapid horizontal gene transfer in the consumer's kitchens which allowed it to become more transmissible after the salad left the plant, thus causing the expanded geographic spread.
• C. Food production and distribution: centralized processing and wide distribution networks can spread a contaminated product broadly and cause simultaneous outbreaks over wide geographic areas.
• D. Complacency: the population had previously stopped washing salad because a historical program had eliminated foodborne pathogens, and that lack of individual washing alone explains multi-state spread without any role for centralized distribution.

Answer: (C)