By Michael Nedelman, CNN

(CNN) — The US Centers for Disease Control and Prevention is telling doctors to be on the lookout for a polio-like illness that tends to peak in the late summer and early fall, shedding new light on the illness in a report released Tuesday.

Last year saw the highest number of cases of acute flaccid myelitis so far — 233 confirmed cases in 41 states. It tends to spike between August and October every other year, including outbreaks in 2014 and 2016 with 120 and 149 cases, respectively. So far this year, there have been 11 confirmed cases in eight states, including Pennsylvania, out of 57 patients under investigation.

Acute flaccid myelitis is a rare but serious condition that affects the nervous system — specifically, the area of the spinal cord called gray matter. The CDC estimates it affects fewer than 1 or 2 in a million children each year in the United States. There are no known treatments or ways to prevent it, the agency says, though experts say early physical therapy or rehab may be key.

“AFM is a devastating illness for patients and their families,” Dr. Anne Schuchat, CDC’s principal deputy director, told reporters Tuesday. “We know families are facing uncertainties when it comes to their child’s recovery from AFM, and we want parents to know that we are keeping their children front and center and working with our partners to better understand this illness, its risk factors and ways to treat and prevent it.”

Experts don’t fully understand the long-term consequences or why some patients recover quickly while others continue to experience paralysis and weakness, which can cause respiratory failure in severe cases.

The agency is hoping that doctors learn to quickly recognize and report the illness to health officials to help unravel the mysteries of AFM. According to the new report, doctors collected specimens between two and seven days after limb weakness started, but it took weeks for cases to be reported to the CDC.

“This delay hampers our ability to understand the causes of AFM,” Dr. Tom Clark, deputy director of the CDC’s Division of Viral Diseases, told reporters Tuesday. “We believe that recognizing AFM early is critical and can lead to better patient management.”

Scientists have long thought that a virus — specifically an enterovirus — was most likely at the root of AFM.

According to the CDC’s latest report, nearly all patients with confirmed AFM had been sick in the month leading up to the onset of paralysis, including 92% with fever or a respiratory illness. Of the respiratory specimens collected, 44% tested positive for an enterovirus or rhinovirus.

Enteroviruses are common; they cause about 10 million to 15 million infections a year in the United States, according to the CDC. Though they’re around all year long, they’re most common in the summer and fall, which is also when AFM peaks.

Typically, enteroviruses cause people to have cold-like symptoms such as fever, runny nose and body aches, and recovery is easy.

It’s unclear why a relatively small number of people develop paralysis after an enterovirus infection. Even within the same family, several siblings can develop the cold-like symptoms, but only one may become paralyzed.

“We are monitoring AFM trends and the clinical presentation, conducting research to identify possible risk factors, using advanced lab testing and research to understand how viral infections may lead to AFM, and tracking long-term outcomes of AFM patients,” Clark said in a statement Tuesday.

The CDC escalated its response to the illness, more than doubling the number of staffers working on AFM late last year. This followed earlier criticisms from parents of children with AFM and some of the CDC’s own medical advisers toward the agency for being slow to respond to the outbreak.

CDC Director Dr. Robert Redfield called for the formation of an AFM task force in November, which he said Tuesday has yielded “important contributions … in helping us get closer to critical answers.”

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