Cases similar to Tunan's son are spreading to children around the age of four nationwide. The agency had a teleconference with health providers slated for later today, and it provided a toolkit for diagnosing AFM, along with instructions for reporting suspected cases.
The long-term effects of the disease are also unknown.
It shows distinct abnormalities of the spinal cord gray matter on magnetic resonance imaging (MRI).
Since 2014, 386 cases have been confirmed, the CDC said on Tuesday. Only one death has been reported, which involved an AFM illness reported in 2017.
Since officials have been unable so far to determine how the disease spreads, they are starting to count suspected cases as well as confirmed to better anticipate increases over the coming months. Several cases have been linked to enteroviruses or other germs, but officials have not been able to find a cause for the majority of these cases.
"Right now, we know that poliovirus is not the cause of these AFM cases", Messonnier said.
"As a parent myself I understand what it's like to be scared for your child", Messonnier said. Symptoms can also include facial droop and weakness, difficulty moving the eyes, drooping eyelids or difficulty swallowing and slurred speech.
To give parents, healthcare workers, and public health officials a look at what to expect, she said the CDC will report suspected cases this year, as well as confirmed ones. According to the CDC, AFM is a serious condition commonly seen in children that causes weakness in the arms or legs. Then in 2016, there were 149 confirmed cases.
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It's too early to know whether the total for 2018 will surpass those previous years. But the data reported Tuesday represents "a substantially larger number than in previous months this year", Messonnier said. It affects fewer than one in a million people each year across the country, the CDC estimates. "Parents need to know that AFM is rare even with the increase in cases we are seeing now". The symptoms tend to occur about a week after they had a fever and respiratory illness.
Lacking an established cause, health officials confirm cases through a review of brain scans and symptoms.
But the agency doesn't know who may be at higher risk nor why they may be at higher risk.
Health officials call the condition acute flaccid myelitis (AFM).
The CDC is investigating the cases and monitoring the disease, and encourages people to prevent the disease by staying up to date on vaccines, washing hands and protecting against mosquito bites.
The illness is called acute flaccid myelitis or AFM. Officials would not say what states they lived in, but cases have been reported in New York, Minnesota, Illinois, Colorado, and Washington.
CNN reached out to health departments in every state; 48 states responded, plus the District of Columbia.