CUCUTA, Colombia — The doctor taps Zulay Balza’s knees with a hammer and she doesn’t feel a thing. She can’t squeeze his outstretched fingers or shut her eyelids. Her face is partially paralyzed.

“The weakness started in my legs and climbed upward. The face was last. After three days, I couldn’t walk,” said Balza, 49. “My legs felt like rags.”

Balza is a patient at the public University Hospital in Cucuta, at the epicenter of the Colombian outbreak of the mosquito-borne Zika virus. Only Brazil has more cases.

Two weeks ago, she came under assault by Guillain-Barre a rare and sometimes fatal affliction that is the Western world’s most common cause of general paralysis.

Alarm over the Zika epidemic spreading across the Americas has been chiefly over birth defects, but front line physicians believe a surge in Guillain-Barre cases may also be related.

The World Health Organization says Guillain-Barre cases are on the rise in Brazil, Colombia, El Salvador, Suriname and Venezuela, all hit hard by Zika, though a link remains unproven.

The auto-immune disorder historically strikes only one or two people in 100,000. About one in 20 of those cases ends in death, and it is frightful.

“I thought my body was going to explode,” said Balza, sitting on her hospital bed and apparently over the worst.

Guillain-Barre attacks skeletal muscular nerves as if they were a foreign enemy. Fine motor skills rapidly erode, arms and legs tingle and weaken to numbness. Patients lose their balance, their speech. In rare cases, they require ventilators to stay alive.

The syndrome typically strikes after a bacterial or viral infection, such as influenza, HIV or dengue, though its cause can’t always be determined.

Dr. Jairo Lizarazo, the neurologist treating Balza, has seen cases increase more than tenfold since December — 30 cases in all — in this muggy city bordering Venezuela. Like Balza, many patients never showed the characteristic symptoms of Zika — fever, rashes, joint pain and conjunctivitis. Four in five don’t.

He’s convinced the virus boosts susceptibility to Guillain-Barre.

“It’s an epidemiological association,” said Lizarazo. “We don’t know exactly how it works. But it’s there, for sure.”

Associated cases confirmed or suspected based on clinical evidence number in the hundreds. Guillain-Barre cases believed to be linked to the virus have killed three people in recent weeks in Colombia and health officials have attributed another three Guillian-Barre deaths in Venezuela to suspected Zika infections.