Neurological conditions like it have a variety of causes, such as viruses, environmental toxins and genetic disorders.
The CDC has received more than 120 reports of patients under investigation for the illness this year and confirmed at least 62 cases.
Messonnier said the CDC has definitively ruled out polio - which causes a similar set of symptoms - as the cause.
Roughly 90 percent of the cases so far have involved children, who suffer from muscle weakness or paralysis that impacts face, neck, back or limbs.
Fox 2/KPLR 11's Dan Gray talked to an infectious disease specialist who said the disease isn't new but should be considered a growing health concern. The paralysis sets in about a week after the children have come down with fever and respiratory illness.
Because officials don't know the cause of AFM, they can't recommend a specific way to prevent it.
Lacking an established cause, health officials have confirmed cases through a review of brain scans and symptoms.
"This is a mystery and we haven't solved it yet, so we have to be thinking broadly", said Messonnier, adding, "You'd think that we would finding a single agent and that's not what we're finding". CDC has tested every stool specimen from every AFM patient.
In September, Pardo-Villamizar and his colleagues published a follow-up of 16 patients stricken in 2016 that concluded that "the majority of children with AFM have limited motor recovery and continued disability".
Messonnier stressed that while she understands how frightening this situation is for parents, they should remember that the infections are, in fact, rare.
Vaccines and hand-washing help protect against the spread of viral illnesses, while insect repellent helps ward off the mosquitoes that transmit West Nile.
Any child experiencing weakness or loss of muscle tone in the arms and legs should be examined immediately.
Officials said they will be conducting additional analysis on this year's cases.
"Early intervention is definitely always helpful".
"We know this can be frightening for parents", Messonnier said. "You don't need an very bad lot of paralyzed children to make this an important problem".
Geography also does not appear to be a factor, with the confirmed cases spread across 22 of the 50 U.S. states. In 2015 and 2017, the counts of reported illnesses were far lower.