The link between a respiratory virus called enterovirus D68 and a polio-like illness has been bolstered by new research showing a spike in both the virus and reports of acute flaccid myelitis in children in 2018, a new government report suggests.

The report released Thursday by the Centers for Disease Control and Prevention reinforces previous research that the virus strikes every other year and in the late summer and early fall.

In 2018, 358 of 2,579 tested patients were positive for EV-D68. In the same year, the CDC confirmed 223 cases of the polio-like illness, acute flaccid myelitis. That compares to 2017, when researchers found the virus in two out of 2,433 patients with acute respiratory illness who were tested.

The average age of patients who tested positive for EV-D68 was 3. Almost 60 percent of patients with EV-D68 were male, according to the CDC.

The new report came from a network which tracks acute respiratory illness among children and teens under the age of 18 at seven U.S. medical centers: Cincinnati; Houston; Kansas City, Mo.; Nashville; Pittsburgh; Rochester, N.Y.; and Seattle.

#DYK? Enterovirus D68 is one of many known enteroviruses, and can cause mild to severe respiratory illness, or no symptoms at all. New in @CDCMMWR: substantial increase in pediatric cases of acute respiratory illness-associated EV-D68 from 2017 to 2018. https://t.co/udY1AonU0H pic.twitter.com/Pp3SK0FkuP — CDC (@CDCgov) March 28, 2019

This spike in cases in 2018 suggests there will be fewer infections with EV-D68 and fewer cases of AFM in 2019, Dr. Matthew Elrick, pediatric neurologist and AFM expert at the Johns Hopkins School of Medicine told NBC News. “But just because it’s been every other year so far that doesn’t mean it will continue to be exactly every other year."

AFM first attracted attention in 2014 when 120 cases were reported nationally.

The link with EV-D68 has been hard to prove conclusively because the symptoms of AFM often show up after the virus has cleared the respiratory tract, Elrick said. And tests have shown several viruses such as coxsackievirus A16 and enterovirus A71 in the bodies of patients, according to earlier CDC reports.

“But the fact that outbreaks of EV-D68 and AFM come together — there were spikes in the number of cases in 2014, 2016 and 2018 — and the fact that EV-D68 shows up in respiratory specimens of roughly half of AFM patients certainly adds support to the fact that it’s likely to be the leading causative agent,” said Elrick.

Experts suspect that in many of the children infected with EV-D68, symptoms are limited to the respiratory tract.

“There’s probably a very small percentage of kids who get an infection and develop AFM as a result,” Elrick said. “Most commonly they are asymptomatic or have a mild cold.”

Scientists are still learning how AFM is similar to polio. While both viruses can cause paralysis, they affect different parts of the body. AFM is very rare, but that could change, Elrick said. Polio didn’t immediately become a widespread problem and EV-D68 is in the same family of viruses, he added.

Some experts have suggested that the enterovirus could mutate to become more infectious. As yet, “there is no evidence to suggest that changes in the virus are associated with transmission or infectiousness,” said Dr. Susan Gerber, a CDC researcher in the division of viral diseases at the National Center for Immunization and Respiratory Diseases, told NBC News in an email.

The CDC’s data shows an increasing number of cases in the even numbered years:

In 2017, CDC received information for 35 confirmed cases of AFM in 16 states.

In 2016, CDC received information for 149 confirmed cases of AFM in 39 states and DC.

In 2015, CDC received information for 22 confirmed cases of AFM in 17 states.

From August to December 2014, CDC received information for 120 people confirmed cases of AFM in 34 states.

Elrick said he found the number of children diagnosed with AFM in 2018 to be concerning, since it falls in line with the trend of increasing cases shown in previous years.