And for now, it's hard to say if 2018 will equal or surpass spikes seen in 2014 and 2016, Messonnier said, adding that state and federal health officials haven't finished the whole diagnostic algorithm for numerous cases reported over the past several weeks.
The federal Centers for Disease Control and Prevention is investigating 127 cases of possible AFM, including 62 that have been confirmed in 22 states this year. Those officials are probing another 65 illnesses in those states. All told 386 cases of AFM have been confirmed since 2014, more than 90 percent in children younger than 18. Most of the cases have occurred in children.
"Despite a lot of investigation by CDC and our partners, AFM remains a mystery disease", said Dr. Nancy Messonnier, director of the National Center for Immunization and Respiratory Diseases for the CDC.
The CDC said the condition isn't new, "but the increase in cases we saw starting in 2014 is new".
Data collected since the establishment of standardized surveillance helped with the identification of another increase in reports nationally during 2016 and has provided additional valuable information on the clinical presentation to help better characterize the clinical features, epidemiology, and short-term outcomes of cases of AFM.
AFM may be caused by other viruses, including enterovirus, environmental toxins and a condition in which the body's immune system attacks and destroys tissue that it mistakes for foreign material, Messonnier said: "This is a mystery so far, and we haven't solved it yet, so we have to be thinking broadly".
Some victims have been infected with viruses, but researchers have been unable to identify a single virus responsible for all cases.
Acute flaccid myelitis (AFM) affects the gray matter in the spinal cord, causing sudden muscle weakness and a loss of reflexes. Some also experience facial droop, difficulty moving the eyes, drooping eyelids, difficulty swallowing, or slurred speech. The illness can lead to serious complications - including paralysis or respiratory failure - and requires immediate medical attention.More news: Boko Haram militants kill kidnapped aid worker
"We know this can be frightening for parents", Messonnier said.
Cases have been on the rise since 2014.
The CDC said it doesn't know how long symptoms of the disease will last for patients.
That's when we spoke with the families of 4-year-old Camdyn Carr, who's now fighting the disease, and 7-year-old Sebastian Bottomley, who previously fought AFM.
Messonnier said some patients diagnosed with AFM have recovered quickly while others continue to struggle with paralysis.
The cases this year seem to be spread across much of the country, as were the earlier two waves. "As a parent myself, I understand what it's like to be scared for your child", she said.
Because officials don't know the cause of AFM, they can't recommend a specific way to prevent it. Numbers dropped drastically in 2015 and 2017 - to 22 and 33, respectively - but were back up again in 2016 at 149.
The CDC is actively investigating and monitoring disease activity and recommends taking standard prevention measures such as hand-washing, protecting oneself from mosquito bites and staying up-to-date on vaccinations.