But, because doctors believe AFM is caused by respiratory viruses, it's unclear whether recent measures to mitigate the spread of COVID-19 might have an impact on the outbreak of AFM predicted this fall.
The coronavirus is not known to lead to AFM, but other viruses called enteroviruses, specifically D68 and A71, are strongly suspected. They spread the same way COVID-19 does: close contact with infected people, and by touching surfaces that have the virus on them, and then touching the mouth, nose or eyes.
"With all of the social distancing measures, mask wearing and improved hand hygiene, I hope that that will really decrease enterovirus circulation," Dr. Janell Routh, the AFM team lead in the CDC's Division of Viral Diseases, said.
AFM affects the nervous system and causes limb weakness, most often in young children. It is rare: 633 cases have been confirmed in the United States since the CDC started tracking them in 2014.
Though cases have been reported every year since then, larger outbreaks of AFM have followed an every-other-year pattern, spiking in 2014, 2016 and 2018.
That's why scientists are predicting another significant rise in cases in 2020.
"We normally see cases between August and November. About 86% of our cases fall between those months," Routh told NBC News. "But this year is unprecedented. Anything can happen."
What's more, enteroviruses "will be circulating at the same time as flu and other infectious diseases, including COVID-19," CDC Director Dr. Robert Redfield said during a call Tuesday with reporters.
So far in 2020, 16 cases have been confirmed in the U.S. One of those patients, who was over age 18, died of the illness.
AFM almost always starts with a mild head cold: cough, runny nose, fever. About a week later, the patient develops weakness in one or more limbs, and may have trouble walking. It's still unclear whether certain conditions may increase a child's risk, but cases are often reported in otherwise healthy children.
"AFM can progress quickly, and patients can become paralyzed in the course of hours or days," Redfield said.
Cases are confirmed with MRIs, which show lesions on the spinal cord. There is no specific treatment, other than supportive care and physical therapy to help patients regain strength.
Patients can have a variety of outcomes. "We definitely see children recovery their limb function," Routh said. "But we also know that there are children who remain severely paralyzed and require ventilation and supportive nursing care around the clock."
On Tuesday, the CDC released a report on the 238 AFM cases recorded in 2018. Most patients were around age 5. Half of those young patients needed treatment in the intensive care unit, and nearly a quarter were placed on a ventilator.
There were a few significant symptoms just before the onset of weakness. Routh said kids tended to report a "striking headache, and neck and back pain." Those may be early warning signs to call a physician, she said, even during the ongoing coronavirus pandemic.
"We have seen a decline in emergency room visits and even parents taking children to pediatric visits," she said. "With COVID on everyone's mind, it's particularly important to get this message out now."
"Limb weakness, accompanied by neck or back pain, particularly after a viral illness, should all be triggers to seek medical care immediately."
This story was originally published on NBC News.