Last week, the C.D.C. confirmed the presence of the Zika virus in a baby born with an unusually small head and brain in Hawaii — the first case of brain damage linked to the virus in the United States. (The mother had been in Brazil while she was pregnant.) But the C.D.C. added that it had not found an increase in Guillain-Barré cases in any American territory, including Puerto Rico.

Camila Bogaz, a spokeswoman for Brazil’s Health Ministry, cited a recent study by the Federal University of Pernambuco showing that some patients with Guillain-Barré syndrome had previously been infected with Zika. But she, too, cautioned that the connection between Zika and Guillain-Barré was still under investigation.

Outside Brazil, other countries around Latin America where Zika is spreading are reporting an increase in cases of Guillain-Barré, including Colombia and Venezuela. This month, El Salvador said it was seeing a surge in Guillain-Barré cases. It normally averages about 14 per month, but between Dec. 1 and Jan. 6 it recorded 46, two of them fatal.

Of the 22 patients for whom medical histories were available, about half remembered having had the fever and rash that are the most common symptoms of Zika, between a week and two weeks before their paralysis began.

Most people with Guillain-Barré recover, but their struggle is often harrowing. Patients in Brazil described a creeping inability in their limbs to feel textures, heat and pain, along with sensations of tingling in parts of their body. In severe cases, they can become almost completely paralyzed — conscious but unable to speak or move, as if trapped inside their bodies — and can go into cardiac arrest or comas.

“It felt like I was drowning in a sea of mud,” said Geraldo da Silva, 43, a construction worker in the town of Murici in northeast Brazil who was hospitalized for 10 days in an intensive care unit after doctors discovered he had Guillain-Barré. “I became motionless and thought I would die. All of this happened just a few days after I had Zika.”