Thirty people have been diagnosed with a rare paralyzing condition caused by Zika virus infection in Puerto Rico, the territory's health department said Thursday.
And Centers for Disease Control and Prevention director Dr. Thomas Frieden says he expects even more cases of Guillain-Barre syndrome in Puerto Rico because the virus is infecting so much of the population.
"We think there will be as many as 200 additional cases, given the overall number of infections there," Frieden told NBC News.
"We expect there will be a fair number of cases."
He said Guillain-Barre is usually seen in about every 5,000 to 10,000 infections like Zika.
Puerto Rico's health secretary, Ana Rius Armendariz, reported the 30 cases in an update to the territory's government Thursday.
Guillain-Barre is a rare complication of many different types of infections. It's a usually temporary paralysis that can become serious if people's breathing muscles become affected. With treatment, most patients recover.
Zika's usually not serious but it can very rarely cause fatal complications - a Puerto Rico man died in April. The biggest risk is to pregnant women, whose fetuses can become infected if their mothers catch it.
Related: How Zika Can Kill
Zika's now known to cause profound birth defects and there is no cure for the fetus once affected. Babies can be born with microcephaly - a small head caused by the destruction of brain tissue. They can also die in the womb, or they can suffer other defects from malformed limbs to subtle brain damage.
The CDC has reported 529 pregnancies affected by Zika in the continental U.S. and another 691 in U.S. territories, mostly Puerto Rico.
In the U.S., 16 babies have been born with birth defects caused by Zika and five have died, miscarried or been aborted because of Zika damage.
The spread's so bad in Puerto Rico that the U.S. declared a state of emergency there last week.
But Clarisa Jimenez, the CEO of the Puerto Rico Hotel & Tourism Association, accused CDC of exaggerating the threat.
"From the very beginning the numbers that were given were based on projections. The reality is that as of today, less than half of 1 percent of the population has the virus," Jimenez told CNBC Thursday.
Frieden pushed back. "Unfortunately, when babies start to be born with microcephaly, that will change perceptions," he said.
"We projected, based on the chikungunya experience, that Zika might infect a quarter of the population in the first year and it is very much on track to do that," Frieden said.
Chikungunya is closely related to Zika and carried by the same Aedes mosquitoes. It's hardly ever deadly but extremely painful, so people remember getting it. Zika, on the other hand, doesn't cause symptoms in most people.
"People in Puerto Rico are saying Zika isn't like chikungunya," Frieden said. But he said it's just that people notice it less.
Related: Answering Your Questions About Zika
"There is a lot we don't know about Zika," Frieden added.
Armendariz said about a quarter of people infected in Puerto Rico are noticing Zika symptoms, including headache, rash, conjunctivitis and joint pain.
She's advising everyone to use mosquito repellent.
"I always say use it like perfume," she told NBC News. "Nothing replaces that."
"Wear light clothes, long sleeves, pants," she said, adding women should avoid sandals even though it's hot. She said pregnant women should make sure their partners use condoms. "You don't know if he has been bitten," she said. "If you are pregnant, protect yourself."