One-quarter of the island’s 3.5 million people will probably get the Zika virus within a year, according to the Centers for Disease Control and Prevention, and eventually 80 percent or more may be infected.

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SAN JUAN, Puerto Rico — The Zika virus has begun spreading through Puerto Rico, now the United States’ front line in a looming epidemic.

The outbreak is expected to be worse in Puerto Rico than anywhere else in the country. The island, a warm, wet paradise veined with gritty poverty, is the ideal environment for the mosquitoes carrying the virus.

The landscape is littered with abandoned houses and discarded tires that are perfect breeding grounds for the insects. Some homes and schools lack window screens and air-conditioning, exposing residents to almost constant bites.

The economy is in shambles, and thousands of civic workers needed to fight mosquitoes have been laid off. The chemical most often used against the adult pests no longer works, and the one needed to control their larvae has been pulled from the market by regulators.

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One-quarter of the island’s 3.5 million people will probably get the Zika virus within a year, according to the Centers for Disease Control and Prevention (CDC), and eventually 80 percent or more may be infected.

“I’m very concerned,” Dr. Tom Frieden, the CDC director, said in an interview after a recent three-day visit to Puerto Rico. “There could be thousands of infections of pregnant women this year.”

The epidemic is unfolding in one of the country’s most popular vacation destinations, where planes and cruise ships deliver thousands of tourists daily. Anyone could carry the virus back home, seeding a mosquito-borne outbreak or transmitting it sexually.

Health officials in Puerto Rico have begun intensive efforts to stop the virus, which has been linked to abnormally small heads and brain damage in babies born to infected mothers, and to paralysis in adults.

Trucks are rumbling through communities, shrouding them in insecticide. Schools are being outfitted with screens to protect children, and hundreds of thousands of old tires have been retrieved and disposed of.

Officials have warned scores of island towns that they must clean up the detritus in which standing water collects, incubating new mosquitoes. And the CDC is preparing to spend tens of millions of dollars to blunt the spread of the virus.

Officials are not optimistic they will succeed. “I’m not going to oversell this,” said Dr. Johnny Rullán, who was the territory’s secretary of health for 31 years and has come out of retirement to advise the governor. “It’s not a perfect world. We’ll do as much as we can.”

In desperation, officials are focusing much of their effort on protecting one group in particular, pregnant women, instead of trying to shield the entire population.

“Zika has shaken us to our core,” said Dr. Brenda Rivera, chief of the Puerto Rico Health Department’s response.

The territory’s first Zika case was confirmed in December, and the governor declared a health emergency Feb. 5. Even though there are just 249 confirmed cases — 24 of them in pregnant women — doctors assume many more are going unreported.

At the moment, the island is where northeast Brazil was in late 2014, when cases of a “mystery disease” with a rash, low fever and red eyes began appearing. By spring 2015, cases of Zika infection began overwhelming hospital emergency rooms.

A temporary paralysis, Guillain-Barré syndrome, struck a few patients soon afterward. Babies with misshapen heads and brain damage — a defect called microcephaly — did not appear until that August.

In Puerto Rico, there has thus far been only one case of Guillain-Barré paralysis linked to Zika infection. If microcephaly appears, it will probably not be until fall, Rullán said.

In some respects, Puerto Rico is lucky. Virtually every doctor and nurse here has been alerted and knows what to look for.

The CDC has a permanent lab on the island and is importing staff and equipment to do 100,000 blood tests a year, five times as many as it can do now.

The agency has committed $25 million to the fight and has asked Congress for $225 million more as part of President Obama’s $1.8 billion Zika package, which is stalled in Congress.

Perhaps most important, the government operates dozens of mother-and-child nutrition clinics, which are used by more than 90 percent of pregnant women here.

Clinic staff members are calling pregnant clients, inviting them to attend 20-minute lectures about the Zika virus. About 5,000 women have attended.

“Ladies, this year’s fragrance is DEET,” said Ismarie Morales, head nutritionist at the Women, Infants, and Children clinic in Carolina, a suburb of San Juan, as she held up a green can of repellent at a recent class. “We all should smell like this.”

Her seven listeners reacted with varying degrees of concern.

Highly visible fogging by trucks is what most people associate with government mosquito-fighting. But permethrin, the insecticidal fogger used for years, may be useless. Tests at several sites have found that its ability to kill mosquitoes has significantly faded, so the island will have to choose a new pesticide and retrain workers to use it.

Moreover, by a “very inconvenient coincidence,” the Environmental Protection Agency (EPA) has effectively banned the chemical used here to kill juvenile mosquitoes, Audrey Lenhart, a CDC entomologist, noted. The chemical, temephos, has been in use since 1965 and works, she said. But it is not profitable, so when the agency demanded safety data costing about $3 million to gather, the manufacturers decided instead to quit making it.

Mosquitoes are not the only mode of Zika transmission, however. Health authorities have also been forced to deal with an unexpected twist: the discovery that sex spreads the virus.

The government has frozen the price of condoms, threatening stores with fines of up to $10,000 if they raise them.