MCKINNEY, Texas » From the cool shade of the dugout, Marcus Robinson watched his son Kingston come up to bat for the Horned Frogs.

Kingston, 4, aimed his bat at the T-ball and connected, powering a hit to mid-field. The parents and siblings who filled the bleachers at Plano’s Archgate Park cheered as Kingston, tall for his age and slim, trotted off to first base.

"I’m still amazed he’s out here doing anything," Marcus, shaking his head and squinting against the midafternoon sun, told The Dallas Morning News.

One year ago, Kingston lost his ability to walk, stand and raise his right arm after coming down with a sore throat and fever.

He spent 20 days in the hospital fighting to regain his health. At one point, he told his parents he thought he’d never walk again and his father, too, began to lose hope.

Kingston, who is from McKinney, is one of more than 115 children in 34 states who became paralyzed last year after developing what seemed like an ordinary cold.

Experts at the U.S. Centers for Disease Control and Prevention are still unsure of what caused the illnesses, which peaked in September before falling off.

The cases coincided with the largest-ever outbreak of enterovirus D68, a virus that is related to polio but typically causes only flu-like symptoms. The agency is tracking cases like Kingston’s and bracing for the possibility of a new spike in paralysis cases this summer and fall, when enterovirus D68 usually makes its rounds.

On June 27, 2014, Kingston’s mother, Courtney, took him to the doctor for a fever and achy throat. His pediatrician diagnosed strep throat and bronchiolitis, an inflammation of the airways leading to the lungs, and put him on antibiotics.

A few days later, Kingston’s fever came down, but he still felt unwell.

He seemed unsteady on his feet. After tripping and falling, he told Courtney, "Mommy, I don’t have any energy."

On the night of July 1, Courtney and Marcus had their first inkling that something more serious was happening. Marcus was in Kingston’s bedroom changing him into pajamas. When Marcus tried to pull off Kingston’s shirt, he found that his son couldn’t raise his right arm.

Marcus called Courtney into the room to have a look. They decided to take him to an aftercare clinic. Doctors there took X-rays. They said Kingston probably had a muscle tear, and they placed his right arm in a sling.

Concerned, Courtney took Kingston the next day to his regular doctor, where his symptoms grew worse.

He forgot the name of his favorite toy from the movie "Planes" — Ripslinger — and asked Courtney several times to repeat it.

Then, as Courtney and the doctor were talking, Kingston, who had been standing up and playing, lost his balance and fell.

"’Here, let me help you up," his doctor said, walking over to him.

Once he was up, he fell again.

His doctor told Courtney to take Kingston to the emergency room at Children’s Medical Center Plano. She called the hospital to let them know one of her patients was on his way.

Kingston’s initial round of X-rays and blood tests came back negative, so the Plano team transferred him by ambulance to Children’s Medical Center Dallas.

In Dallas, Kingston could still walk, though he was weak. Doctors told Courtney and Marcus that they were trying to rule out childhood stroke.

Courtney knew that something major was going on, but when doctors used the word "stroke," she felt stunned.

It would be 24 more hours before experts would have a better picture of what was happening to her son.

As early as 2013, doctors at Children’s Medical Center Dallas had noticed some unusual symptoms in the patients they were seeing.

Dr. Benjamin Greenberg, a neurologist at Children’s Health and at UT Southwestern Medical Center, is an expert on rare forms of paralysis. Patients with muscle weakness fly from across North America to see him.

He noticed that a small but growing group of patients were coming in with limbs that were floppy — not stiff as in many cases of paralysis — and MRI scans of their spines showed a distinct pattern.

Greenberg specializes in transverse myelitis, a type of spinal cord inflammation, and runs one of only two clinics in the country dedicated to the condition. Transverse myelitis is exceedingly rare, affecting between 1 and 8 people out of a million. Officially, it is an "orphan disease," one with so few patients that drumming up research funding and attracting scientists to the field is a challenge.

Transverse myelitis — myelitis means inflammation of the spinal cord — has traditionally been thought to damage only the cord’s white matter, which insulates the nerves traveling down the spinal cord from the brain. Doctors believe it sets in when a person’s immune system mistakenly attacks a patient’s body, possibly in response to an infection.

But the newer group of patients Greenberg had started noticing had damage to a different part of their cord: the gray matter, which carries signals from the spinal cord to the muscles.

Physicians in other states were noticing a similar phenomenon. In February 2014, doctors in California reported that "a mysterious polio-like illness" had been found in five children. All had lost muscle function in an arm, leg or in multiple limbs. All had inflammation in the gray matter of their spinal cords, and many had recently had colds.

More concerning was that, unlike patients with transverse myelitis, few of these newer patients improved with treatment.

On July 3, 2014 — at 11:30 a.m., Courtney wrote in her iPhone — doctors performed an MRI scan on Kingston and took a sample of his spinal fluid.

By evening, they had the results: The gray matter in Kingston’s spinal cord was inflamed, disrupting signals to his muscles.

To Courtney and Marcus, the diagnosis came as a relief after days of uncertainty. "I think we came away from it, like, ‘OK, they know what it is. Let’s go!’" said Marcus.

Following the protocol for transverse myelitis, the condition most closely related to Kingston’s that physicians knew how to treat, doctors started Kingston on a five-day course of steroids to ease the inflammation. He would need physical therapy, too.

The next morning, Marcus and Courtney awoke in Kingston’s hospital room. They had sent their 6-month-old daughter, Savannah, to live with Courtney’s sister while they nursed Kingston through his illness. Courtney had taken a leave from work.

Marcus walked to Kingston’s bed and helped him up, thinking they would start on some of the exercises the physical therapists gave him. But when Marcus let go of his son, Kingston crumpled to the ground. He could no longer walk or stand.

Marcus and Courtney looked at each other and tried not to panic.

The Robinsons struggled to keep their nerves in check through several days of steroid treatments and seemingly fruitless physical therapy sessions.

Kingston dreaded physical therapy. He grew increasingly frustrated as his limbs refused to do what he asked of them. He was afraid of falling and getting hurt.

Doctors resorted to a range of incentives to keep his spirits up: They gave him Incredible Hulk hands to wear while working out and brought Zippy, a hospital robot, to his room to distract him from the stress of trying to stand and walk.

By the fourth day, Kingston was no better.

Marcus, who had always considered himself the spiritual leader of the Robinson household, felt his faith faltering.

He turned to Courtney for support.

Sitting in the hospital hallway one day, Marcus asked her, "Do you think he’ll get back to 100 percent?"

To Marcus’ surprise, Courtney didn’t hesitate.

"Yes," she said. She knew she was going to do everything in her power to help her son walk again.

After five days of steroids, Greenberg came to Kingston’s hospital room late in the evening. The therapy had failed, but Greenberg remained optimistic about Kingston’s prospects.

The group decided on a new course of treatment known as IVIG, or intravenous immunoglobulin. The treatment floods the patient with "good" antibodies from donors. The healthy antibodies are thought to block the action of "bad" antibodies, rogue agents of the immune system that attack a patient’s own body.

Soon, Kingston turned a corner.

Courtney steered her son into the hospital’s playroom aboard a red wagon, which served as Kingston’s main mode of transportation. She held him up so he could practice standing, and he said, "Mommy, don’t touch me. I can walk by myself."

He took four or five steps.

He continued to improve. After two weeks at Children’s, Greenberg and his team transferred Kingston to Our Children’s House at Baylor Dallas, where he did in-patient physical and occupational therapy for one week and then went home.

In September 2014, the CDC issued a bulletin warning physicians about a spike in limb weakness among children. While it’s typical for a handful of patients per year to develop unexplained limb weakness, doctors were seeing far more cases than usual.

The CDC initially called the new disorder acute flaccid paralysis, then changed the name to acute flaccid myelitis (AFM) and asked doctors to report any new cases to the agency. By April, the CDC had counted 118 cases across the U.S.

In a statement to The Dallas Morning News, the CDC said that the cause of AFM remains unknown but that there is a strong correlation between AFM and the outbreak of enterovirus D68, which sickened more than 1,100 children in 49 states last year.

A study published in June in the journal Lancet Infectious Diseases found that 12 of 25 patients diagnosed with AFM in California and Colorado between January 2012 and November 2014 had D68 in their nasal passages.

Kingston wasn’t tested, because he was diagnosed before the test for D68 became more widely available. Greenberg says there is a greater than 50 percent chance that Kingston, too, had D68, possibly in combination with strep throat.

In June, researchers at the University of Virginia School of Medicine identified a new virus that may have been responsible for some of the paralysis cases: enterovirus C105, which is even more closely related to polio than D68.

They found the virus in the nasal passages of a 6-year-old girl who developed weakness in her right arm last October after a cold. Enterovirus C105 was first identified in the Republic of Congo and in Peru in 2010 and has been linked with at least one previous case of paralysis.

For now, the CDC believes the outbreak of acute flaccid myelitis is over.

"Our current data suggest that cases of childhood AFM have returned to ‘expected’ rates," the CDC said in its statement. "We are maintaining vigilance for another possible ‘spike’ in cases in the summer/fall of 2015, however, in order to maintain the highest level of public health preparedness for this devastating illness."

As for D68, the CDC says it received its first reports of the virus in August 2014, so it may be too early in the season to tell if it will be back. So far this year, the agency has yet to detect a single case of D68.

Kingston’s recovery from AFM was unusually fast and complete.

In April, the CDC reported that only two of the children diagnosed with AFM since August 2014 had made a full recovery. Two-thirds showed some improvement. One-third reported no improvement.

Greenberg believes this is because of several factors, including Kingston’s age. "I think the 3- to 6-year-olds in some respects do better with recovery," said Greenberg. "They are old enough to have walked already, but they’re still pretty neurologically plastic."

Kingston also had less inflammation in his spinal cord than many other patients, and his inflammation was lower down, affecting mainly his lower limbs and not the muscles that control breathing.

Bryan Sotelo, a 12-year-old from Allen, developed AFM soon after Kingston and was also treated by Greenberg and his team. He was almost completely paralyzed and spent more than one month in intensive care and on a respirator.

During a June office visit with Greenberg, Bryan, dressed in a Captain America T-shirt, shorts and sneakers, showed significant improvement. Though still in a wheelchair, he has recovered the use of his left arm and the ability to move his feet. A few days before, he stood up for the first time in nearly a year.

Greenberg told him and his mother, Yadira Garcilazo, that movement was returning to his limbs. He said that doctors hope to soon be able to close Bryan’s tracheostomy, a hole in his wind pipe that helps him breathe and allows him to use a ventilator at night. Greenberg was optimistic that Bryan would walk again.

"The AFM kids are definitely not responding in the early days to the therapy we’re doing," said Greenberg. "But every few months, as they’re coming back to clinic, there is steady improvement. It’s taking a lot longer, and we don’t know what the plateau is going to be."

Greenberg speculates that recovery from AFM might be slower because a virus attacks the spinal cord directly, like polio. But most treatments are aimed not at stopping the virus but at curbing the body’s immune response, which is likely to blame in classic cases of transverse myelitis.

Doctors are working hard to crack the mystery of acute flaccid myelitis. Greenberg believes the CDC should declare myelitis — whether it affects the gray matter, the white matter, or both — a reportable condition, like measles and tuberculosis.

"We’d be ahead of the curve and not behind the curve, if we are trying to pick up an upswing," he said.

He said that the story of AFM is still evolving. He expressed particular concern over a handful of paralysis cases on which he consulted in December and January. "In a period of eight weeks, my phone rang four times, with the exact same (MRI) images sent to my computer," he said.

Each patient was from a different part of the U.S. and Canada, and they were between the ages of 17 and 23 — older than the AFM kids, whose median age was 7. Each MRI scan showed gray-matter inflammation that resembled AFM, but with more significant swelling. Each patient had become quadriplegic virtually overnight. None immediately responded to treatment.

Because the illnesses struck at a time when D68 was not circulating, Greenberg believes a different virus or a different trigger is to blame. His theory is that a group of viruses is circulating that causes benign symptoms in the vast majority of people. But, in an exquisitely small number of patients, for reasons no one yet understands, they cause devastating symptoms.

Greenberg added the four cases to a national database he recently started at UT Southwestern that tracks myelitis cases. Comparing different types of spinal cord inflammation helps his team better understand the disease in its many manifestations.

He currently has about 40 patients enrolled, of whom about a dozen had AFM. The goal is to see how patients fare over time and what treatments are most and least effective.

The CDC, too, is continuing its surveillance and has devoted a specific team to AFM.

"I firmly believe we will sort these things out," said Greenberg. "It takes way more time than any of us want."

Back home last August, Kingston’s — and his family’s — recovery continued. After taking a leave from work, Courtney changed positions so she could better balance work with taking Kingston to outpatient physical therapy and other appointments. His baby sister, Savannah, came home after living with her aunt for weeks.

"My sister saved our lives," said Courtney.

While Kingston could now walk on his own, he couldn’t yet run and remained unsteady on his feet. To celebrate his release from the hospital, Kingston’s soccer team invited him to attend its last practice of the season.

Kingston wobbled up a hill on weak legs to reach the playing field and fell — a big change from when he was one of the fastest players on his team.

Marcus began shadowing Kingston on the field to make sure he didn’t hurt himself. He followed his son with outstretched arms, ready to catch him.

But Kingston waved off Marcus. "Dad, just let me play," he said.

After that first practice, Kingston felt dejected. He told Marcus he no longer wanted to play soccer, his favorite sport, because he couldn’t keep up with his friends.

Marcus felt heartbroken.

But, over the next several months, Kingston grew stronger and stronger.

At the hospital, he had learned to never give up.

Kingston’s father repeatedly told him that hard work pays off — a phrase Kingston repeated as: "Hard work plays off."

The lessons sunk in, and Kingston eventually caught up to his friends.

His parents say he is still a little bit slower than before his illness, and his right arm is noticeably thinner than his left, but the muscles are coming back.

Kingston says he feels as good as new.

In his living room, after the T-ball game, Kingston ran around, jumped on the couch, flipped upside down into headstands and asked for more snacks.

"Is there anything your friends can do that you can’t do?" his father asked from the kitchen.

"No," said Kingston.

"Is there anything you can’t do?"

He thought for a moment.

"I can’t pick up real cars," he said.