At this point, researchers are working to find the connective tissue among some of the largest measles outbreaks in the U.S. within the past year, including the ones centered in West Texas, Utah, Arizona, and South Carolina. Technically, the epidemics still could have been caused by separate reintroductions of measles from at least one international source. But “that’s a hard stretch,” Robert Bednarczyk, a global-health researcher and epidemiologist at Emory’s Rollins School of Public Health, told me. The most likely and so far best-supported scenario, he said, is also the simplest one—that the virus spread so fiercely and quickly through these communities that it was able to hitch a ride elsewhere in the country when infected people traveled.