The Centers for Disease Control and Prevention has issued an advisory to health care providers about a recent uptick in severe respiratory illness requiring hospitalization in children, urging them to consider a type of enterovirus as a possible cause.

In July and August, hospitals and medical professionals in several parts of the United States reported an increase in infections caused by enterovirus D68, or EV-D68, which often causes mild, coldlike symptoms but can also lead to more severe respiratory illness. The number of detected cases is now the highest it has been since a 2018 wave of seasonal infections. A representative for the C.D.C. confirmed there have been 84 cases of EV-D68 from March through Aug. 4.

The recent alert was directed toward doctors and hospitals because — in addition to causing respiratory symptoms — EV-D68 has been linked to acute flaccid myelitis, or A.F.M., a rare but serious neurological complication that can lead to permanent paralysis or death. More than 90 percent of A.F.M. cases in the United States have been in young children.

The C.D.C. has not received increased reports of A.F.M. cases so far this year, but because spikes in EV-D68 infections tend to precede cases of A.F.M., the agency hopes to put pediatricians and other frontline health care providers around the country on alert. It is important for diagnostic tests to be done as soon as possible after a child develops symptoms.

“Increased vigilance for A.F.M. in the coming weeks will be essential,” the C.D.C. cautioned.

Non-polio enteroviruses such as EV-D68 are very common, causing up to 15 million infections every year in the United States. There are more than 100 known types, and they tend to spread in the late summer and the early fall, though infection happens year-round.

“They circulate every year,” said Dr. Alejandro Jordan Villegas, a pediatric infectious disease physician with Orlando Health Arnold Palmer Hospital for Children. “Some years we see more cases than other years, but these are not something new.”

Most people who are infected are asymptomatic or have mild, coldlike symptoms, such as fever, runny nose, coughing, sneezing or body aches. Two of the better-known enteroviruses — enterovirus A71 and Coxsackie virus A6 — can cause hand, foot and mouth disease, a mild contagious infection that is very common in children under 5.

The C.D.C. alert also notes that some of the patients recently hospitalized with respiratory illness had tested positive for rhinovirus, which most often causes the common cold.

EV-D68 was first identified in 1962, though the C.D.C. has been tracking it closely only since 2014, when it caused a nationwide outbreak of respiratory illness. That year, the agency confirmed 1,395 cases in 49 states and the District of Columbia, though it believes there were probably many thousands of additional unconfirmed mild cases.

“In most people, it is an acute viral illness that comes and goes without incident; in the very select few, it can cause neurologic symptoms like this acute flaccid myelitis,” said Dr. Amy Arrington, medical director of the special isolation unit and section chief of global biologic preparedness at Texas Children’s Hospital. “But it’s very rare.”

There have been 693 confirmed cases of A.F.M. since the C.D.C. began tracking the virus in 2014, including 120 that year. Roughly 10 percent of EV-D68 cases in 2014 caused A.F.M., though that may be an overestimate given that many enterovirus infections are thought to go unreported. There have since been additional spikes in A.F.M. cases in 2016 and 2018 — increases the C.D.C. believes were caused by EV-D68, though experts are still investigating how the virus affects the nervous system and why a small number of people develop A.F.M. after a viral infection.

There have been 14 confirmed cases of A.F.M. in the United States this year.

The most common symptoms of A.F.M. are sudden onset of arm or leg weakness, loss of muscle tone and loss of reflexes. In addition to those warning signs, the C.D.C. also tells parents to seek medical attention if children develop symptoms like difficulty swallowing or slurred speech, drooping eyelids or facial weakness, particularly after they have had a respiratory illness.

Non-polio enteroviruses spread when an infected person coughs, sneezes or touches a surface that is then touched by others. So the best practices many families, schools and child care centers have leaned on during the Covid-19 pandemic can also help prevent the spread of common seasonal viruses, including EV-D68, this fall.

“The main, main, main thing is hand washing,” Dr. Villegas said, noting that soap and water are generally believed to be superior to hand sanitizer when it comes to infection control.

He also emphasized the importance of “respiratory etiquette.” “If you are sick, try not to expose others,” Dr. Villegas urged. Children with asthma may have higher risk of severe symptoms from EV-D68.

Parents should be on the lookout for signs of serious illness in their children but should also remember that the purpose of these types of C.D.C. health alerts — which the agency issues regularly, and which are sometimes picked up by mainstream news outlets — is to put pediatricians and other frontline health care providers on alert. There is no specific treatment for A.F.M., but doctors may recommend physical or occupational therapy.

“It’s incredibly rare, but it is something that you don’t want to miss when you have a child coming in with these neurologic symptoms, because getting that child checked in and treated and having rehab start as early as possible is really critical,” Dr. Arrington said.