In reality, measles never went away.

At the petri-dish level, the virus—one of the most stable, unchanging strains there is—looks just the way it did in the pre-vaccination era. Measles remains one of the most infectious illnesses on the planet. The virus stays active and contagious in the air for up to two hours, and can be transmitted from an infected person for up to four days before and after a rash appears.

The stability of the measles virus is also what makes its vaccine so effective. "Oftentimes viruses mutate a lot, like the influenza virus, but this virus is very stable," said Cody Meissner, a professor of pediatrics at Tufts University School of Medicine. "There's really only one strain of the measles virus."

And although the United States successfully eradicated it—meaning that despite occasional outbreaks, measles doesn't move through the population continuously—there's no guarantee it will stay that way. That's what happened in the United Kingdom, where measles was wiped out but is now endemic again.

"It can definitely come back," Meissner told me. "And then because this is probably the most infectious of all the known viruses or illnesses—we say that about 90 percent of people who are not immune and who are exposed to measles will get it—that's a higher number than for any illness, even influenza. It's one of the most infectious or transmissible viruses that we're aware of."

Measles is already one of the leading causes of death among young children worldwide. About 400 people die from the virus each day—that's about 16 deaths every hour, according to the WHO. "It's a very severe disease," Meissner told me. "It's not a mild illness like mumps or even chickenpox. This is a much more severe sort of illness." Even those who survive the virus can suffer brain swelling, pneumonia, deafness, and other permanent complications. And in the United States, measles seems to be making a comeback. Today there are 67 confirmed measles cases in the United States, most of them linked to Disneyland in California. This puts the United States on pace to eclipse incidences of measles in 2014, which was already the worst year for measles since 1994, when there were 958 cases reported to the CDC. Last year there were 644 cases of measles reported in 27 states.

"It is very easy for entire communities to be exposed when an unvaccinated individual is infected and brings it into that group," said Roberta DeBiasi, the chief of Pediatric Infectious Diseases at Children's National Medical Center.

U.S. Measles Cases by Year

CDC

"The problem is people not getting vaccinated," said Jane Seward, deputy director of the Division of Viral Diseases at the Centers for Disease Control and Prevention. "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."