North America’s three biggest measles outbreaks have grown to a total of more than 2,500 known cases. Three people have died in the U.S. and one in Mexico. Mexican and U.S. officials say the strains of measles spreading in Canada match the other large outbreaks. One official says the “virus was imported, traveling country to country.” Cities on the borders, like El Paso, Texas, and Ciudad Juarez, Mexico, are informally working together to stop the spread. A recent World Health Organization report said measles activity in the Americas region is 11 times as much as the same timeframe last year and that the risk level is “high” compared to the rest of the world’s “moderate” level.