What is the most common etiology of pasteurized milk outbreaks, accounting for about 44%?

• A. Salmonella
• B. Listeria
• C. Campylobacter
• D. Norovirus

Answer: (D)

The key idea is that pasteurized products can still become implicated in outbreaks if contamination occurs after processing, and viruses can be a major culprit in that scenario. Norovirus is extremely contagious and can be introduced into milk after pasteurization through infected handlers, contaminated surfaces or equipment, or contaminated cleaning water. Its high stability in the environment and very low infectious dose mean that even small amounts can cause illness in many people, so when outbreaks are traced to pasteurized milk, Norovirus often emerges as the leading cause. That’s why it accounts for a large share of these cases—about 44%. The other organisms listed are more commonly linked to raw milk or to failures during pasteurization or post-process contamination in ways that don't explain the relative frequency seen with pasteurized-milk outbreaks.