Johann Castro Hernandez is an 18-year-old college kid who loves playing soccer. Only these days, he can barely lift his legs, let alone kick a ball. His body looks weirdly thin, with no muscle tone. His movements are slow and tentative.

For the past month, he has been in the intensive care unit of a health facility called Clinica Norte in Cucuta, a city in northeast Colombia where doctors have diagnosed him with Guillain-Barre syndrome, a rare neurological condition that can leave people temporarily paralyzed for weeks.

Castro sits at a table looking into a hand mirror as his mother pushes down on his forehead. It's a physical therapy exercise to help him regain use of his facial muscles. He's supposed to furrow his brow — then release.

"Down, down and release," his mother instructs. Castro focuses hard on his image in the mirror. But his face stays put.

"It's like my brain sends the order but the muscles don't follow it," he says.

In the room next door, another Guillain-Barre patient is in even worse shape. He's Jose Cordero, a 58-year-old day laborer. He lies in his hospital bed, breathing tubes leading into his mouth. His sister, Ana Tilcia Cordero, stands by his side, staring down at him sadly. She touches his leg gently. "Can you feel it?" she asks. "No?"

Cordero fixes his eyes on her and strains to shake his head. That's all he can manage.

The head of the ICU, Dr. Arturo Arias, pokes his head into the room.

"Hi, Jose," he says. "Look over here — at me!" He smiles — encouraging. "Don't worry," he tells Cordero. "You'll get better soon."

But as the doctor leaves the room, his expression is dark.

Cordero's situation is serious, he says. "So far, he hasn't been responding to treatment."

When you have Guillain-Barre, your immune system basically starts attacking your nerve cells. For most people it starts with a tingling or numbness in the arms and legs, like when limbs fall asleep. There's weakness and pain. For some that's where it ends. But many other patients quickly deteriorate further.

"It gets harder and harder to move," says Arias.

Eventually a patient is paralyzed and can't even get out of bed. In the worst cases, the paralysis is so severe that a machine is needed to breathe. There's a risk of death from complications like pneumonia or heart attacks. Treatments like plasmapheresis — a procedure that removes antibodies from your blood — can help. Ultimately patients do recover the ability to breathe. But it can take months to walk again. Some people never do.

Guillain-Barre is a rare disorder. The U.S., for example, records 3,000 to 6,000 cases a year. At his ICU, Arias sees about a case a year. So when a patient came in with the syndrome the first week of January, he wasn't too concerned.

The following week, two more patients arrived.

"Two more patients in the same month," Arias thought. "That's strange."

When the fourth case showed up, he decided to call his friends at other ICUs around town.

"Hi, how many Guillain-Barre cases have you had lately?" he asked them.

The answers stunned him.

Four, said one colleague. Another had five. Another eight.

And there was something else. Practically every one of these patients — including both Castro the college student and Jose Cordero — said that shortly before they developed Guillain-Barre, they had come down with the telltale signs of a Zika infection: rashes, pink eye, fever.

Arias brightens at the sight of a visitor to the clinic. Dr. Juan Manuel Anaya has just arrived to help Arias launch a study to puzzle out the Zika connection. He's here from Colombia's capital, Bogota, where he directs a center for autoimmune disease research at the Universidad del Rosario.

"We think that Zika is involved in almost all the [surge of] cases [of Guillain-Barre] physicians in Cucuta have seen," Anaya explains.

And it's not just here. Across Colombia there have been almost 100 Guillain-Barre cases linked to Zika since the outbreak began last fall — about twice the usual number. Other countries where Zika is spreading — Brazil, El Salvador, Venezuela — have seen similar spikes. And it turns out the same thing happened a few years ago during a Zika outbreak in French Polynesia.

We ask Anaya if he thinks Zika is linked to Guillain-Barre. "Mostly, it's an open question," he says.

Still, he adds, viral infections often appear to trigger Guillain-Barre. To prove a specific link with Zika, researchers like Anaya — and there are now quite a number doing similar work in South America — will also have to explain why only a small share of people who get Zika then come down with Guillain-Barre.

"So we're looking for previous exposures to toxins, for instance. Or previous exposure to another virus." They'll also be running genetic tests.

Meanwhile, the patients are struggling to overcome the syndrome. Castro is finishing up his last physical therapy exercise. He's supposed to pucker his lips then release them, in a kiss. His mother, Janina Hernandez, has to push his lips into position. Still, he is at least able to hold them there, then make a kissing sound.

Hernandez looks at her son proudly. Tomorrow he's set to go home. But it's been such a rough road, she says.

"When we first got here he was so worried," she recalls. "I would say to him, 'Get some rest so you can get better.' And he would say, 'No. I'm afraid if I go to sleep I'll stop breathing.' "

Then one morning, that's exactly what happened.

"I started to shake and shake and shake," Castro remembers. "And the doctor said, 'We have to intubate him' " — to insert a breathing tube into his trachea.

By day, Castro tried to keep up a brave face for his mother. Alone, at night, "I'm not gonna lie," he says, "there was more than one night that I cried. I got to a point where I thought, 'Am I going to be stuck like this?' "

Now, he's optimistic. "I'm going to do these exercises every day," he says, "until I can play [soccer] again."

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