For decades, vaccinations have held the mumps at bay. But right now, health officials are seeing the worst outbreak in at least 10 years.

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The Centers for Disease Control and Prevention reports mumps in all but four states, with nearly 4,300 infections reported as of early December. Among the hardest-hit states are Arkansas, Iowa, Illinois, Indiana, Massachusetts, New York, and Oklahoma. Arkansas appears to be the epicenter — with nearly 2,200 cases in mostly school-aged children.

Because the viral illness is incredibly contagious, it’s important to know the signs. The two most-common symptoms are puffy cheeks and a swollen jaw. A fever, headache, muscle aches, tiredness, and loss of appetite are also signs of infection. Symptoms typically appear 16 to 18 days after infection, but the time period can be even longer than that.

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Mumps can be spread by coughing, sneezing, talking, and sharing anything — cups, eating utensils, toys, and touching objects or surfaces with unwashed hands that are then touched by others.

Why are we seeing so many more cases these days? Many people blame the anti-vaccine movement, in which parents avoid routine vaccinations for their kids out of fear the vaccines may do more harm than good.

Meghan May, an associate professor of microbiology and infectious diseases at the University of New England College of Medicine, isn’t so sure that’s the case — or at least the sole factor. May’s research is focused on pathogen evolution. She has been researching the virus.

Related: A Dire Emergency in Human Health 

“In my opinion, this is related to the mumps virus diversifying. For a long time, the prevailing wisdom has been that mumps virus doesn’t rapidly evolve the way something like influenza does, and therefore we weren’t really worried about vaccine ‘escape,'” May told LifeZette.

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The mumps virus doesn’t evolve as fast as the flu, May added. But the strains being seen right now are genetically distinct from the vaccine strain.

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“Perhaps children are not quite protected enough against the new strains. Between waning immunity and genetic diversity — perhaps we have the perfect storm for mumps cases to start reappearing in people living in close quarters, such as college students, whose immunity is many years old,” she said.

And more mumps cases in younger adults means that more mumps virus is going to be circulating in the population. Unvaccinated children, said May, are thus at enormous risk — and can introduce the virus into their own social circles where other unvaccinated children, either by their parents’ choice or a medical limitation, would then be at risk.

“It’s a bit of a time bomb,” said May.

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May stresses clear communication about vaccinations between parents and pediatricians. She also said it’s critical the long-term consequences of diseases like mumps are discussed. “Mumps isn’t always a ‘harmless childhood’ disease — some patients who recover wind up with permanent effects. Mumps can cause encephalitis — inflammation of the brain — which can be either fatal or leave permanent damage. Even patients with standard mumps presentations can suffer permanent hearing loss or damage to the testicles that can impair male fertility.”

Related: Mumps Make the Rounds of College Campuses

May encourages everyone to follow vaccination recommendations. The current outbreak in vaccinated people suggests that either genetic diversity or waning immunity — or both — is contributing to a shorter-term immunity than previously thought.