Which of the following definitions most accurately distinguishes an emerging infectious disease from a reemerging infectious disease?

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 hospital uses a particular broad‑spectrum antibiotic extensively for ten years. Over time clinicians observe breakthrough infections by a formerly susceptible bacterial species. Which evolutionary mechanism most plausibly explains this phenotypic change and what immediate stewardship action would be most appropriate?

Measles has an R0 of approximately 15. Calculate the herd immunity threshold (HIT) and interpret whether a community with 90% MMR vaccination coverage is above or below the threshold and thus at risk for sustained transmission. Show calculations.

Which statement correctly summarizes the primary comparative roles of the CDC (national) and WHO (global) in outbreak response?