Surveillance is how health authorities notice. Indicator-based systems analyse routinely reported data; event-based systems scan informal sources for signals; genomic and environmental methods, including wastewater monitoring, add a further layer. In practice most outbreaks are still first detected by a clinician or laboratory noticing something unusual.
This topic covers what those systems can and cannot see, the obligations the International Health Regulations place on states, and the delay between a signal and a verified event.
How outbreaks are detected and controlled: surveillance systems, verification, epidemiological investigation, IHR obligations, containment measures and response gaps.
How global immunization programs work: herd immunity, cold chain, delivery systems, WHO and Gavi roles, vaccine safety monitoring and barriers to coverage.