Final answer:
Questioning individuals who did not fall ill serves as a control in an epidemiological study to trace the source of Cyclospora infection and understand how cyclosporiasis is spread.
Step-by-step explanation:
You would question people who did not become ill about possible sources of infection with Cyclospora to identify a control group in an epidemiological study. This approach is vital to pinpoint the common factor among those who were infected and to contrast their exposure to that of the non-infected group. By comparing foods eaten, water sources, and overall hygiene practices between the two groups, researchers may uncover patterns that help identify how the cyclosporiasis infection was transmitted. Understanding that some individuals can carry a pathogen without showing symptoms, as demonstrated in the historical examples of asymptomatic carriers of cholera and typhoid, reinforces the importance of this comprehensive approach to tracing infection sources and controlling outbreaks.