“The problem is people not getting vaccinated,” said Jane Seward, deputy director of the Division of Viral Diseases at the Centers for Disease Control. “The vast majority of our cases every single year are unvaccinated people who choose not to be vaccinated. They are living in a family who are unvaccinated and they have friends who are unvaccinated. They might go to a school with a high proportion of people who are unvaccinated.”

That’s true in parts of California, which has developed a reputation for communities where anti-vaccine attitudes thrive. Some 8 percent of kindergarteners enrolled in California schools have exemptions from the measles vaccine, according to CDC data. Some California schools have exemption rates as high as 43 percent, according to data compiled by the data visualization platform Silk…

To complicate matters further, there’s an entire generation of doctors in the United States who have never treated a measles patient, or even seen a case in person. “The success in general of the vaccination program does mean that younger physicians have never seen a case, and they don’t necessarily think about it at all,” Seward told me. “The other challenge, which is nobody’s fault at all, is that measles presents early-on looking just like an upper respiratory infection with fever. But it can be contagious before the rash. At that stage it’s not distinguishable from the flu or other respiratory viruses.”