Which of the following describes passive surveillance?

• A. Data are collected actively with field investigations.
• B. It is always complete and timely.
• C. Relies on sentinel sites.
• D. Diseases are reported by vets, physicians, laboratories, or others.

Answer: (D)

Passive surveillance gathers information when cases are reported to public health authorities through routine channels, rather than investigators actively going out to look for cases. The defining feature is reliance on reports from frontline sources—veterinarians, physicians, laboratories, or other reporters—these entities notify health authorities about suspected or confirmed cases as part of normal operations. Because it depends on voluntary or mandated reporting, passive surveillance tends to be less complete and can have delays, especially if cases are not diagnosed or reported promptly.

This helps differentiate it from active surveillance, where public health teams actively seek out cases through field investigations; and from systems that emphasize sentinel sites, which are specific locations used to monitor trends rather than a description of the overall reporting approach. Also, the notion that reporting is always complete and timely doesn’t fit passive surveillance, since underreporting and lag times are common.