Even before the latest numbers in the Ohio outbreak were tallied, the CDC warned that there were already more U.S. measles cases in the January-April period this year than there have been since 1996.
The cluster of cases in Ohio - as with an outbreak of 58 people in California earlier this year - originated with people returning from the Philippines, which is currently experiencing what the CDC called an "explosive outbreak" of measles. (There were more than 20,000 suspected cases in the Philippines in just January and February this year.)
In the United States, measles was considered eliminated in 2000. But while vaccines have helped eradicate measles around the globe, first-world countries like the U.S. and the U.K. have seen an uptick in exemptions: those who choose not to vaccinate their children for religious, personal, or philosophical reasons.
These exemptions let diseases that should be left in the history books creep back into society.
"The number of people who are choosing non-medical exemptions... is certainly in the tens of thousands," Dr. Paul Offit, a pediatrician and vaccine expert told Business Insider in March. "You are seeing what you expect to see when more and more people choose not to vaccinate."
The vast majority of those infected with measles in the U.S. in recent years have been willfully unvaccinated. Others are unvaccinated because they are too young or because their immune systems are compromised and they cannot be vaccinated. That group - the most vulnerable - is protected by "herd immunity" when everyone around them is vaccinated, but those protections crumble when more and more people opt out of vaccination.
Measles is extremely contagious: "Almost everyone who has not had [the] vaccine will get measles if they are exposed to the measles virus," the CDC warns.
As Phil Plait put it in Slate:
Measles is not a disease we should be screwing around with. 30 percent of cases develop complications like pneumonia, diarrhea, or ear infections. One in five children who contract it are hospitalized. One in a thousand will get encephalitis. One or two out of a thousand will die from it. Yes, die. From a disease that is essentially wholly preventable with a vaccine.
Here's a look at where measles has cropped up in the U.S. since 2008 - the current pace of infections puts 2014 on pace to beat 2011 and surpass 1996:
