Measles

measles is one of the most transmissible viral diseases known, with a basic reproduction number of roughly 12 to 18 in a fully unvaccinated population, meaning a single infected person can pass the virus to more than a dozen others in a crowd with no immunity. It spreads through airborne droplets that can linger in a room's air for up to two hours after an infected person leaves, well beyond direct contact.

The two-dose MMR vaccine (measles, mumps and rubella) is highly effective, and herd immunity requires roughly 95 percent coverage to reliably break chains of transmission; below that threshold, clusters can seed rapidly in under-vaccinated pockets even where national vaccination rates look adequate on average.

measles has resurged sharply across the Americas in 2026. The Pan American Health Organization recorded 20,521 cases in the first five months of the year, with Mexico at 10,920 and Guatemala at 6,209, roughly four times 2025's total, prompting surveillance recommendations ahead of the FIFA World Cup.

Pennsylvania alone reached 88 confirmed cases in 2026, its highest annual total in three decades, concentrated in Lancaster and Lebanon counties. On 30 June, two Chester County residents became the first cases confirmed near Philadelphia, a World Cup host city, since the winter. No case has been linked to the tournament's crowds; public-health teams are watching an under-vaccinated regional outbreak drift toward a host city rather than a tournament-driven event. WHO's R&D Blueprint separately named measles among the pathogens covered by its Q1 2026 Paramyxovirus roadmap, alongside Nipah and Hendra.