It seemed that 6-year-old Spencer Hill was battling symptoms of a common cold until suddenly he started struggling to walk and move his arms.

Then he couldn’t write and his legs began giving out.

“You see your child, who likes to run and play,” Serena Hill, the Tennessee boy’s mother, told WRCB. “All of a sudden, ‘Mommy I can’t zip my pants. I can’t write.’ Then you see him fall and his legs give out. It is absolutely terrifying.”

Spencer is among the 62 confirmed cases across 22 states of acute flaccid myelitis (AFM), with the Centers for Disease Control and Prevention (CDC) investigating 65 additional more. According to his mother, he’s been in a children’s hospital in Atlanta since last month and has spent the last three weeks working on his fine motor skills while receiving intense steroid infusions to stop the spread of paralysis.

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About 90 percent of the confirmed cases involve children who have suffered muscle weakness or paralysis, including in the face, neck, back or limbs. The symptoms tend to appear about a week after exhibiting a fever or respiratory illness.

On Tuesday, CDC officials said they haven’t found a cause in the recent spike of cases, which is similar to patterns reported in 2014 and 2016. On its website, the agency lists viruses, environmental toxins and genetic disorders as possible causes of AFM. It also lists West Nile virus, adenoviruses, poliovirus, and non-polio enteroviruses as potential suspects, but while some have been detected in recent paralysis cases, it hasn’t been found in others.

“This is a mystery so far,” the CDC’s Dr. Nancy Messonnier said.

Many children eventually recover from the illness, with treatments varying depending on their symptoms and severity of illness, but others may not.

Carter Roberts, who according to his family was a fully vaccinated and healthy child, was struck by the virus at age 3 in 2016. Carter’s family said that following his diagnosis he spent nearly 200 days in the hospital, including over 100 days in intensive physical therapy, but he remained fully dependent on a ventilator unable to move on his own, with his prognosis unlikely to change.

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On Sept. 22, nearly 2.5 years after his diagnosis, and on the same day that he received a toe-controlled motorized wheelchair that would help him navigate kindergarten, Carter died.

Carter Roberts died on Sept. 22, about 2.5 years after being diagnosed with AFM. (GoFundMe)

“We are so sad to announce that Carter passed away last night around 8 p.m.,” a post on the “Carter’s Coalition” Facebook page said. “Although we miss him more than words can express, he is free of his machines and tubes. He is free to run, climb and play like he did before AFM. He is surrounded by love and peace in heaven. Our family is forever changed by the almost 6 years we had with him on this year. His voice, spirit and joy will be terribly missed but never forgotten.”

The CDC did not disclose which of the 22 states have confirmed cases but said that most of the cases began in August and September. Messonnier said that, in general, parents can help protect their children from disease by practicing hand washing and applying insect repellent to protect against mosquito bites.

The Associated Press contributed to this report.