Public health officials had said that enterovirus D68 first looks like a cold or flu and is rarely fatal. Those facts have been called into question after a young boy died in his sleep with only mild symptoms.

Enterovirus D68 (EV-D68) has claimed its first victim: 4-year-old Eli Waller of Hamilton Township, New Jersey.

The boy, the youngest of fraternal triplets, died during the night on Sept. 25 after showing early signs of what his parents thought might be pink eye.

Officials investigating his death first determined that he had been infected with EV-D68 but weren’t sure what role it played in his death. The Mercer County coroner’s office later found that the virus had caused Eli’s death, a finding that local health officials have made public.

The Centers for Disease Control and Prevention (CDC) is investigating four other deaths of patients who were infected with EV-D68. A Rhode Island child died after contracting the enterovirus and a staph bacterial infection.

Before Eli Waller’s case, the virus had never proven fatal.

EV-D68 first appeared in California in 1962, but the enterovirus has caused few illnesses until this year. The CDC first identified EV-D68 as the culprit behind a rash of severe respiratory illness in children in early September.

Adults, who have more mature immune systems, are almost never sickened by enteroviruses.

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Public advisories indicated that the virus first causes symptoms similar to a cold or flu, but that some children go on to have trouble breathing. That’s the time parents should take them to the hospital, the CDC said.

But Eli Waller wasn’t even sick when he went to bed, Hamilton Township health officer Jeff Plunkett told Healthline. The boy’s parents found him unresponsive in the morning.

The death seems to have taken the CDC somewhat by surprise.

“Even though Enterovirus 68 is quite common this year, it is mainly causing very mild illness and not this most horrific presentation that just occurred,” Dr. Anne Schuchat, director of the CDC’s National Center for Immunization and Respiratory Diseases, told CNN. “A very rapidly progressive death in one’s sleep is surprising but not completely unanticipated.”

The CDC did not respond to Healthline’s request for comment.

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The CDC has confirmed 664 cases of the virus in the United States, affecting all but five states. New Jersey now has 14 cases, according to a Department of Health spokeswoman. The CDC is also performing genetic testing on fluid samples from a second boy who attends the pre-school Eli Waller did, Yardville Elementary. The boy has been very sick with a respiratory illness.

Experts in other states have raised the alarm that EV-D68 may cause neurological illness, including partial paralysis.

The virus is related to the poliovirus, and its behavior may be more similar to polio than experts first thought. In a fraction of cases, polio creeps into the patient’s nervous system, where it can cause paralysis. Tiny clusters of paralysis appearing in the wake of larger clusters of respiratory illness suggest that EV-D68 also affects a small percentage of patients in this way, said Dr. Emmanuelle Waubant, a neurologist at the University of California, San Francisco.

Waubant was among the group of doctors who first hypothesized a connection between EV-D68 and partial paralysis based on a handful of sick children in California in 2012.

Fortunately, expert recommendations for avoiding infection haven’t changed. People are encouraged to wash their hands frequently with soap and water. (Hand sanitizers do not kill enteroviruses.) Washing common surfaces, such as refrigerator door handles, can also help.

If children with respiratory symptoms have trouble breathing or slur their speech, parents should take them to the hospital right away.

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