A measles outbreak hits an under-vaccinated pocket — the unvaccinated infants suffer. Herd immunity needs ~95% measles coverage — below it, the virus finds the unprotected (too-young infants, immunocompromised). The outbreak's geography maps vaccination gaps — epidemiology's report card. Catch-up campaigns + routine strengthening close them. The community's immunity is the infant's only shield — coverage is collective protection.