Herd immunity occurs when enough of a population is immune to a disease -- through vaccination or prior infection -- that sustained transmission becomes unlikely, indirectly protecting those who cannot be vaccinated; the exact immunity threshold needed varies by disease depending on how contagious it is. A standard outbreak investigation follows a sequence: confirm the diagnosis, establish a case definition, count and describe cases by person, place, and time, generate a hypothesis about the source, and implement control measures -- a structure used globally by field epidemiologists and reflected in systems like Africa's Integrated Disease Surveillance and Response (IDSR) framework. In August 2020, the WHO certified Nigeria free of wild poliovirus, marking the African region's certification as free of the wild poliovirus strain -- a milestone reached through decades of mass immunization campaigns, though vaccine-derived poliovirus strains have continued to require separate monitoring and response. Routine immunization programs, often coordinated through a national Expanded Programme on Immunization (EPI), depend on consistent vaccine supply chains (the 'cold chain') and community trust, and a drop in either -- as seen in several countries during COVID-19 service disruptions -- can quickly erode herd immunity gains built up over years.