Carter Roberts was so excited when his new, toe-controlled motorized wheelchair was finally delivered Sept. 22.

The 5-year-old was sure he’d be the envy of his kindergarten classmates as he zoomed around in the custom-built wheelchair.

Carter had been paralyzed from the nose down since 2016, the victim of a rare and puzzling polio-like condition called acute flaccid myelitis.

The wheelchair was going to offer Carter his first taste of autonomy in two years.

“He had been in kindergarten for two weeks and he was loving it,” said his mother, Robin Roberts. “He was in love with a little girl in a rainbow dress and looking forward to driving around.”

Carter tries out his new powerchair on Sept. 22. Carter's Coalition

But just hours after his new wheelchair was delivered, Carter died.

It was just as sudden and unexpected as the day in July of 2016 when, at age 3, the toddler developed AFM, Roberts said. “It was a totally normal day,” Roberts, who lives in Richmond, Virginia, told NBC News this week. “If you had told me it was going to happen, I would have laughed in your face.”

Carter became paralyzed during the second big wave of AFM cases in 2016. The highly unusual neurological condition first caught the country’s attention in 2014, when 120 children were diagnosed with various degrees of muscle weakness and paralysis.

Initially, the wave of AFM cases was linked with a virus that was also circulating, a distant relative of the polio virus called EV-D68. It usually causes mild symptoms typical of viral infections, but for reasons that remain a mystery, EV-D68 can attack the nervous system, causing dizziness, weakness and even paralysis.

Sudden, unexpected onset

Any virus can cause neurological symptoms, and now several others, including one called EV-A71, have also been linked with the paralyzing condition. But the Centers for Disease Control and Prevention has been unable to find any single common cause, and Carter was never diagnosed with any infection at all, Roberts said.

As with all the cases of AFM, the onset is sudden and unexpected.

“In late July, Carter was found in a ball next to his bed pleading for help,” Roberts wrote on Facebook.

“He could not hold up his head or stand. As soon as we went to the ER the doctor thought he had meningitis or a possible stroke.”

Carter Roberts and his two sisters before he was diagnosed with acute flaccid myelitis. Robin Roberts

Within a day, she said, the little boy was paralyzed and needed a ventilator to breathe.

“Literally within 24 hours this perfectly happy, healthy kiddo was a quadriplegic and one of the most serious cases,” Roberts said.

He would never breathe without a ventilator again.

Now doctors fear the country may go through a third wave of AFM cases. So far this year, the CDC has recorded 38 confirmed cases of the condition among children in 16 states. Fourteen have been seen in Colorado, and another six in Minnesota since September. The CDC won’t name the other states. Suspected cases are under investigation in Illinois, Washington and Pennsylvania.

But CDC statistics suggest the cases have come and gone in an every-other-year pattern. Viral diseases rarely follow predictable patterns, but doctors who have treated AFM are bracing for a new wave.

“It’s been circulating in several areas across the U.S. for the past several months,” said Dr. Kevin Messacar of Children’s Hospital Colorado. Enteroviruses such as EV-D68 tend to circulate more in the late summer and early fall, he said.

“We have been working with our state and CDC to create better awareness, better surveillance, and better reporting of this condition,” Messacar said.

AFM is still extremely rare. CDC has confirmed only 362 cases since 2014, although Roberts and the parents of other children say they believe the condition is far more common than that.

Because it is so unusual, parents and doctors may not recognize the danger immediately.

What is becoming disturbingly clear after four years is that although some children recover fully, or almost fully, others do not. Many may require months and years of intensive rehabilitation to regain any semblance of normal movement.

Some, like Carter, may die.

'It sounded like strep'

Braden Scott of The Woodlands, Texas also got sick in July 2016.

“All of our kids had had a cold,” said Rachel Scott, Braden’s mother. But Braden, then 5, didn’t bounce back.

“The cold was still hanging on. He didn’t want to swim with the other kids or go to fireworks. We couldn’t get him to eat anything. We took him to urgent care and they gave him some antibiotics and steroids and fluids and said that should perk him up.”

But it didn’t.

“It sounded like strep,” Rachel Scott said. The clinician who saw Braden assumed he wasn’t swallowing because he had a sore throat. It turned out the muscles used to swallow were becoming paralyzed.

When the little boy couldn’t walk any more, the Scotts took him to the hospital, where he was admitted.

“By the 9th of July his diaphragm had become paralyzed and he stopped breathing,” Scott said. “Luckily, the nurse was in the room.”

Braden had a tube inserted to do his breathing for him. A second tube ran to his stomach to feed him. And the rehab started almost immediately.

“They were trying to get him to sit up,” Scott said. “He couldn’t hold his head up at all.”

After 7 ½ months in the hospital, Braden was sent to a rehabilitation facility in Dallas, where he started three hours of therapy daily. The Scotts, who at the time lived four hours away in Beaumont, Texas, took turns commuting back and forth and caring for their other children.

Then Braden went for six weeks of intensive therapy at the Kennedy Krieger Institute, which specializes in paralysis, at Johns Hopkins Children's Center. Once he came home, the Scotts kept up therapy both in clinics and at home, buying Braden a special exercise bike that stimulates his muscles and encouraging him to walk on a treadmill.

Two years later, Braden still cannot swallow well. Progress is measured in tiny victories.

“At first, we were suctioning the spit out of his mouth every minute or two,” Scott said. “Now he can go a couple of hours without having his mouth suctioned.”

And he has made progress with the two years of physical, occupational and respiratory therapy.

“Initially, the only thing he could move was his left hand,” Scott said. “He could give us a thumbs-up. That was our starting point.”

Now he can walk, sit and stand on his own, although use of his arms is more limited.

“He can use his left arm to write,” Scott said.

Braden also had surgery to move one nerve in his right hand. “That should help him close his hand and maybe hold something,” Scott said.

Is condition more common than reported?

Despite the struggles, Scott knows Braden is lucky.

Carter Roberts also underwent years of respiratory therapy to keep him healthy while using a ventilator. Roberts and her husband took turns sleeping because Carter needed nursing care all night long, and insurance did not pay for a night nurse.

“Carter was approved for tons of nursing, but it was never enough,” Roberts said. “We really had to fight and struggle to find out the intense rehab some of these kids go through.”

Therapy helped, but not nearly as much as the Roberts had hoped it would. He regained the ability to eat. But caring for him was still a full-time job.

The Roberts family also battled with their health insurer for 14 months to get the $32,000 powered wheelchair, and ended up paying for a third of the cost out of pocket. Carter used it for just a few hours.

Roberts and Scott both belong to a Facebook group that includes hundreds of parents with children who have AFM or something similar. They believe the condition is far more common than the CDC says it is, and also believe that quicker treatment may have helped their children.

They swap hints about therapies and treatments, and provide a virtual support group for one another.

Many have tried treatment with antibodies called IVIG or intravenous immunoglobulin treatment, and another approach called plasmapheresis to try to remove harmful components from the blood.

Messacar says there is little evidence these treatments help.

“There are several therapies that have been attempted to try to decrease the inflammation associated with the condition,” he said. “None has proven to be effective.”