Patients with a polio-like condition called acute flaccid myelitis have almost all showed symptoms of viral infections before they developed symptoms, the Centers for Disease Control and Prevention said Tuesday.

That makes viruses a prime suspect in causing the muscle weakness or paralysis that marks the condition, CDC researchers said in a new report. The viruses include one called EV-D68, but also a related virus called EV-A71 and a few others, a CDC research team said.

“Almost all patients with AFM have reported signs and symptoms consistent with viral illness in the weeks preceding limb weakness,” the CDC team, led by Susannah McKay of the CDC’s viral disease division, wrote in their report.

Viruses could directly damage the spinal cord. "Another possibility is that the pathogen triggered an immune response in the body that causes damage to the spinal cord," the CDC's Dr. Nancy Messonnier told reporters in a telephone briefing.

“Clinical, laboratory, and epidemiologic evidence to date suggest a viral association.”

So far this year, the CDC has confirmed 90 cases of acute flaccid myelitis or AFM, and 252 are suspected cases. At least 20 suspected cases have been ruled out as being AFM, the CDC said.

It’s the third year that has seen an increase in the number of cases. There were also cases in 2014 and 2016. That every-other-year pattern fits a viral disease, doctors say.

The CDC has struggled to identify the cause of AFM or to say why it seems to have increased in recent years. It’s still extremely rare, having affected just 400 people, mostly children, since 2014.

Parents and doctors who treat AFM have become increasingly impatient with the CDC. But the agency says the tests that have been done on patients don’t always tell a complete story.

"Oftentimes, despite extensive testing, no pathogen has been detected in spinal fluid," Messonnier said.

"As a mom, I know what it’s like to be scared for your child. Right now, the science doesn't give us an answer."

Messonnier said the agency still must consider all possible causes of the condition, even an as-yet unidentified virus or an environmental toxin. But she said toxins have moved down on the list of suspects, and she said children should receive all recommended vaccines.

"What we don't know is what is triggering AFM," she said. "It may be one of the viruses that we have already detected. It could be another virus that we haven’t detected."

"I’m glad to see they now recognize that AFM is likely viral," Dr. Ken Tyler, a neurologist at the University of Colorado School of Medicine, told NBC News.

Tyler and other researchers have noted that they've found solid evidence that EV-D68 and other viruses can cause AFM. There has also been criticism of CDC's reliance on spinal fluid testing. So far, of the patients tested in 2018, only two have shown evidence of any virus in the spinal cord fluid: one with EV-D68 and one with EV-A71.

Finding a virus in the spinal fluid would be a smoking gun, because healthy people don't usually have germs in their spinal fluid, Messonnier said. Doctors are also advised to take respiratory swabs, but even healthy people will have a mix of germs in respiratory swabs, Messonnier said. "We would find other things they are carrying that aren’t making them sick," she said.

Viruses can cause a range of nervous system side-effects, including AFM but also a similar condition called transverse myelitis, Guillain-Barre syndrome, as well as meningitis and encephalitis.

"Right now in the United States, there are many children who have fever or respiratory illness. We don't know what triggers AFM in some of those patients. That's what we're working hard to figure out," Messonnier told NBC News.

"No pathogen has been consistently found in all of the AFM patients. That's why we're looking not just at EV-D68 and A71 but other things that might be triggering AFM."

Polio is the best-known cause of paralysis and muscle weakness but none of the patients with AFM has tested positive for polio, the CDC says. Polio has been eradicated in the U.S., although it still circulates in Afghanistan and Pakistan.

“Parents and caregivers are urged to seek immediate medical care for a child who develops sudden weakness of the arms or legs,” the CDC advises.

Doctors should ask about respiratory symptoms in any child with sudden onset of muscle weakness, and should take respiratory, stool and spinal fluid samples as soon as possible.

"It's also true that there are certainly other conditions that can cause limb weakness. Parents that are concerned should contact their clinician and get their kid seen quickly," Messonnier advised.

Messonnier said the CDC was working with state health officials to get more complete and timely reporting of suspected cases of AFM. She also said the CDC would work harder to follow up on every AFM case to study and understand its long-term effects on kids.

"As a mom myself, I can certainly understand why parents are worried," she said. "It is important for parents to realize that this still is a relatively rare condition."