Acute flaccid myelitis has a calendar. Cases cluster in late summer and early fall, which is why the CDC's publication of six years of surveillance data this month lands a few weeks before the window opens rather than during it.
The headline finding is a quiet one. Between January 2020 and December 2025, the United States recorded between 17 and 48 AFM cases annually. That is a fraction of what the country saw during the biennial peaks of 2014, 2016, and 2018, when annual counts ran from 120 to 238.
The more interesting part is that the cases stayed low even when they should not have. Enterovirus D68, the virus most closely associated with AFM, circulated at elevated levels in respiratory illness during 2022, 2024 and 2025. In earlier years that pattern reliably produced a spike in paralysis cases. It has not since 2018, and nobody has explained why.
The report also identifies a surveillance gap that matters more than the case count: only about half of AFM patients had stool specimens tested for poliovirus.
What AFM Is and Why It Is Confirmed So Carefully
AFM is a rare neurologic condition that affects the spinal cord's gray matter and predominantly strikes previously healthy children. Weakness comes on fast, within hours to a few days, in one or more limbs, and tends to be more pronounced closer to the trunk than at the hands and feet. In severe cases, it progresses to respiratory failure requiring mechanical ventilation, and it can leave permanent paralysis.
Confirmation involves three things working together. A clinical picture of sudden flaccid limb weakness. An MRI showing a lesion largely restricted to the spinal cord gray matter across one or more segments. And laboratory work on cerebrospinal fluid, respiratory specimens and stool.
The reason the process is exacting is stated plainly in the CDC report: AFM is clinically and radiologically indistinguishable from paralytic poliomyelitis. A child with AFM and a child with polio can look the same to a neurologist and the same on a scan. Only the laboratory separates them.
That is not theoretical. One polio case was identified in New York in 2022, in an unvaccinated adult, with poliovirus subsequently found in local wastewater.
The Testing Gap Is the Actionable Finding
If AFM and polio present identically, then testing stool from every suspected AFM patient is how the country would notice if polio returned. Roughly half of patients in this surveillance period were not tested that way.
The report's recommendation is direct, stating that "stool specimens should be collected and tested from all patients suspected of having AFM." CDC also notes that staying current with polio vaccination reduces the risk of poliovirus infection.
Vaccination coverage among the patients themselves was reassuring, with 75 to 100 percent having received three or more polio doses depending on the year. The gap is in specimen collection, which is a clinical practice issue rather than a parental one.
For families, the practical translation is narrow but real. If a child is hospitalized with sudden limb weakness, stool testing is part of a complete workup, and it is a reasonable thing for a parent to ask about.
Why the Biennial Pattern Broke
The pre-2020 rhythm was striking enough that public health officials planned around it. Peaks in 2014, 2016 and 2018, each tracking waves of EV-D68 respiratory infection, each concentrated in late summer and early fall.
Then it stopped. The pandemic years disrupted circulation of many respiratory viruses, which explains 2020 and 2021 easily enough. It does not explain 2022, 2024 or 2025, when EV-D68 came back, and AFM did not follow.
Several explanations are on the table, and none is established. The circulating strains of EV-D68 may have shifted toward variants less capable of invading the spinal cord. Population immunity among children may have changed in ways that alter who gets a severe outcome. Or the association between EV-D68 and AFM may be more conditional than the earlier pattern suggested.
CDC has been candid about the uncertainty. An earlier surveillance report noted that "when AFM will peak again is unknown." That has not changed with two more years of data.
The honest framing is that the country has had seven quiet years and does not know whether that reflects a durable change or a pause.
What Parents Should Watch For
AFM is genuinely rare. In a country with roughly 73 million children, fewer than 50 cases a year means the odds facing any individual family are very small, and nothing here calls for altered summer plans.
What is worth knowing is the presentation, because speed of recognition affects care. The warning sign is sudden weakness in an arm or leg, often after a mild respiratory illness or fever in the preceding days to weeks. Parents describe an arm that will not lift, a leg that gives out, a child who was running the day before and cannot now.
Other signs include facial droop or weakness, difficulty moving the eyes or drooping eyelids, difficulty swallowing, slurred speech, and neck or back pain. Difficulty breathing is an emergency.
Sudden limb weakness in a child is an emergency department visit, not a next-day appointment. That is true whether or not it turns out to be AFM, since the differential includes several conditions where hours matter.
Ordinary prevention still applies and is not specific to AFM. Handwashing reduces enterovirus transmission, and keeping children current on the recommended immunization schedule, including polio vaccine, is the one clearly protective step available. There is no vaccine for EV-D68 and no specific treatment for AFM. Care is supportive, often intensive, and rehabilitation is prolonged.
What Happens Next
The seasonal window runs roughly from August through October, and CDC maintains ongoing AFM surveillance with case counts reported through state health departments. Whether 2026 stays in the recent range or breaks the pattern will be visible in the fall.
Clinicians are the audience for the report's central recommendation, and whether stool collection rates improve is the measurable thing to watch.
The confirmed finding is six years of consistently low AFM counts alongside an incomplete poliovirus testing practice. The people most affected are young children, though the absolute risk to any one family is very low. The most reasonable action is knowing that sudden limb weakness warrants emergency evaluation and keeping polio vaccination current. The central uncertainty is why the biennial pattern stopped, and whether it will resume.
Frequently Asked Questions
What is acute flaccid myelitis? A rare neurologic condition affecting the spinal cord that causes sudden weakness in one or more limbs, mainly in previously healthy children. It can cause respiratory failure and permanent paralysis.
How many cases occur? Between 17 and 48 per year from 2020 through 2025, down from 120 to 238 during peak years in 2014, 2016, and 2018.
What causes it? It has been associated with enterovirus D68, a common respiratory virus, though the link is not fully understood and other viruses may contribute.
Why does CDC want stool tested? Because AFM is clinically and radiologically indistinguishable from paralytic polio. Stool testing is how poliovirus would be ruled in or out.
Is polio a concern? One polio case was identified in New York in 2022. Most AFM patients in the surveillance period had received three or more polio doses. Staying current on vaccination remains the recommendation.
What symptoms should send us to the ER? Sudden weakness in an arm or leg, facial droop, drooping eyelids, difficulty swallowing or speaking, or any difficulty breathing.
When is the risk highest? Cases have historically clustered in late summer and early fall.
