While enteroviruses – viruses that result in illness that's hard to distinguish from an intense cold – are common, a strain called Enterovirus D68 (also known as EV-D68) has been relatively quiet since its debut in the 1960s. The Centers for Disease Control and Prevention notes that fewer than 100 cases of EV-D68 have been recorded in the United States since it was first identified.

But in the last month, hundreds of children have come down with the virus. While there are no confirmed national numbers, one Denver hospital has reported treating 900 children with the virus, 86 of them requiring hospitalization. So far, 10 states – Colorado, North Carolina, Georgia, Ohio, Iowa, Illinois, Missouri, Kansas, Oklahoma and Kentucky – have contacted the CDC for help in investigating the virus.

What's most notable is the number of children needing hospitalization, say officials.

"It's worse in terms of scope of critically ill children who require intensive care. I would call it unprecedented," Dr. Mary Anne Jackson, a director for infectious diseases at Children's Mercy Hospital, told CNN. "I've practiced for 30 years in pediatrics, and I've never seen anything quite like this," she said.

And health officials aren’t sure what’s going on.

"Why one virus or another crops up in one part of the country or another part of the country from one year to the next is inexplicable," said William Schaffner, head of preventive medicine at Vanderbilt University. "It's a mystery to me."

At first, the virus appears much like a common cold, with sneezing, runny nose and cough. But for some, the cough can become severe and breathing can become difficult. Fever and wheezing may occur as well. Initially, it can be exceedingly difficult to differentiate between a cold and EV-D68. Fever, rash and trouble breathing signal the need to visit the doctor; children with asthma are especially susceptible to severe symptoms.

Like a cold, the virus spreads through contact with those who have it or by touching surfaces that have the virus on them and then touching your face. There’s not a lot you can do to protect yourself beyond following the standard don’t-get-sick measures. The Missouri Department of Health recommends the following steps to help reduce the risk of infection:

  • Wash hands often with soap and water for 20 seconds, especially after changing diapers.
  • Avoid touching eyes, nose and mouth with unwashed hands.
  • Avoid kissing, hugging, and sharing cups or eating utensils with people who are sick.
  • Disinfect frequently touched surfaces, such as toys and doorknobs, especially if someone is sick.
  • Stay home when feeling sick, and obtain consultation from your health care provider.
And while news of a mystery virus invading the nation is unsettling, there is solace in the fact that enteroviruses are not usually deadly.

"All of these folks are going to get better," said Schaffner. "Some of them have more severe illness, such as these children who have developed asthma and are hospitalized. But they should all get better."

CNN reports on the virus in the clip below:

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Enterovirus D68: Mystery virus sweeps across the U.S.
A once-rare respiratory virus known as EV-D68 is sending hundreds of children to the hospital. Here’s what you need to know.