The U.S. was up to 800 cases of measles nationwide on Friday. Two unvaccinated elementary school-aged children died from measles-related illnesses in the epicenter in West Texas, and an adult in New Mexico who was not vaccinated died of a measles-related illness.

When measles began spreading earlier this year in northwest Texas, it didn't take long for the state to reach 50 cases for the first time in three decades. That milestone was a far cry from the 1,097 measles cases that Texas reported back in 1992,