Zika has landed forcefully in America, in one of its poorest and most vulnerable corners, a debt-ridden territory lacking a functioning health-care system, window screens and even a spray that works against the mosquitoes spreading the virus in homes, workplaces, schools and parks.

There are 117 confirmed cases of the virus in Puerto Rico, four times the number at the end of January. The island territory, which has a population of 3.5 million people, is “by far the most affected area” in the United States, Tom Frieden, director of the Centers for Disease Control and Prevention (CDC), said Friday. The number will almost certainly rise sharply in coming weeks, making it ever more likely that the virus will spread to the continental United States.

Dozens of flights move daily between San Juan and Orlando, Washington, New York and other major cities on the mainland. Cruise ships stop here as part of their Caribbean tours. College students will soon head here on spring break.

The growing outbreak has laid bare how deeply Puerto Rico’s debt crisis has cut public programs, including basic health and environmental control services needed to fight the virus. Most homes and public schools — and even some medical facilities — don’t have window screens. A specialist in birth defects at Puerto Rico’s top hospital has trouble obtaining basic supplies, such as toner for his office printer. There are hundreds of abandoned houses — not only in low- and middle-income neighborhoods but also in gated communities — because owners have fled to the mainland as a result of the economic crisis.

At dusk, health department workers spray permethrin in the middle-class neighborhood of Riveras de Cupey in San Juan, Puerto Rico. The government is beefing up anti-mosquito measures as the Zika virus spreads through the island. (Allison Shelley for The Washington Post)

Experts say urgent action is needed before mosquitoes reach their peak with the start of the rainy season in April. Experts from the CDC estimate that 700,000 people — about 20 percent of the population — could be infected across the island by the end of the year, based on previous outbreaks of dengue and chikungunya, related viral diseases.

In response, the CDC has sent 30 experts from its Atlanta headquarters and elsewhere to Puerto Rico, adding to the 70 CDC staff members based here who usually work on dengue fever but now are focusing on Zika. Frieden is expected to visit soon. President Obama’s $1.9 billion emergency Zika request to Congress includes $250 million for Puerto Rico.

“I don’t think we’re going to be able to stop the Zika outbreak,” said Steve Waterman, chief of the CDC’s dengue branch, located on the city’s west side. “There will be a substantial Zika outbreak that will peak in the summer and fall. It’s likely that thousands of pregnant women will be exposed and infected, so that’s why our efforts are focused on protecting as many pregnant women as possible.”

Five of the 117 confirmed cases involve pregnant women. And unlike in the continental United States, where cases are the result of infected travelers to Latin America and elsewhere bringing the virus back home, almost all the cases in Puerto Rico involve people bitten here by infected Aedes aegypti mosquitoes, which also spreads dengue fever and chikungunya.

What you need to know about the Zika virus and how it spreads

Because of the suspected link between Zika and potentially devastating birth defects, authorities are focusing on protecting as many pregnant women as possible. That includes 4,000 expectant mothers living in parts of the island where mosquitoes are spreading the virus. That’s more than one-third of Puerto Rico — primarily San Juan, the northeast and the southern coast.

Only the CDC and Puerto Rico’s health department labs can perform the special Zika testing. The labs expect to run 100,000 tests over the year for pregnant women, five times as many as they handle now, Waterman said. Determining whether someone is infected is complicated because most people don’t show symptoms. It’s also hard for tests to easily differentiate between dengue and Zika infections.

On Monday, authorities in Puerto Rico began distributing free Zika prevention kits to pregnant women that were created by the CDC and the CDC Foundation. The kits include information and tools to help them reduce risk of infection and include repellent, products that kill mosquito larvae, and condoms.

Mosquitoes have ample breeding grounds here. In the Villa Palmeras cemetery in barrio Obrero, a low-income neighborhood in northeastern San Juan, virtually all of the thousands of graves have built-in flower stands where water, and mosquito larvae, collect. There are 109 cemeteries across Puerto Rico and thousands of flower holders.

Mosquito larvae also flourish underground, in water meters and vent pipes of septic tanks, which contain more water than elsewhere in the United States, said Roberto Barrera, a CDC entomologist.

And then there are the mountains of used tires, which mosquitoes flock to, said Johnny Rullan, a former health secretary who is helping the government eliminate breeding sites. Puerto Rico has accumulated more used tires than anywhere else in the United States, experts said. In the past three weeks, temporary collection centers have received more than 561,000 tires.

Elwin Moran, 26, helps pile used tires at a former shoe factory in Humacao, Puerto Rico. The Humacao environmental board is collecting abandoned tires from neighborhoods. (Allison Shelley for The Washington Post)
‘Part of living on the island’

Perhaps the most difficult challenge is changing people’s attitudes and behavior about an ever-present pest that is as much a part of life here as steamy weather and graceful old banyan trees.

“What can I say, it’s part of living on the island,” said José Fernandez, a supervisor at a tire collection center in Humacao, in the southeast.

Emeris Canales Morales, 27, a single mother who is 23 weeks pregnant, lives in a home that overlooks a small cemetery on one side and a fetid canal on the other. Plastic bottles and other trash collect along the banks of the canal. Her windows have no screens. In December, the mosquitoes were biting so hard that she woke up with red welts covering her arms.

At a prenatal clinic for high-risk pregnancies at San Juan’s University Hospital at the Puerto Rico Medical Center, she was among the first to sign up for free Zika screening for women in their first and second trimesters.

Tourists visit two of Puerto Rico’s most famous landmarks — Fort San Felipe del Morro fortress and Santa Maria Magdalena de Pazzis cemetery. Mosquitoes thrive in wet conditions, such as cemeteries. (Allison Shelley for The Washington Post)

She won’t know the results for at least another week. Her first two pregnancies ended in miscarriages because of complications from diabetes. She is hoping for the best this time.

“I haven’t had the fever or the red eyes or the rash,” said Canales, who lives in Loiza, a northeast community that is one of the island’s poorest areas.

But even for pregnant women, it’s hard to stay vigilant against the mosquito.

“When there was chikungunya, we joked about it until everyone had it,” she said. “Until people have the sickness, nobody in Loiza will take it seriously.”

Said Brenda Rivera, chief epidemiologist for Puerto Rico’s health department: “Controlling Zika is going to be a daunting task.” The department is coordinating the island’s response to the public health emergency.

Entomologist Roberto Barrea examines materials at a lab where his team breeds thousands of mosquitoes for research at the CDC’s dengue branch in San Juan. (Allison Shelley for The Washington Post)
Poor and unprepared

Women in Puerto Rico give birth to about 33,000 babies a year. The island has one of the highest teenage birth rates in the United States, and many public high schools have no window screens. The government is estimating how much it would cost to add screens, said Grace Santana, chief of staff to Gov. Alejandro Javier García Padilla.

Nearly half of Puerto Rico lives below the poverty line. The thousands of pastel-hued public housing projects that dot the island don’t have air conditioning. Residents don’t have window screens, in part because they can’t afford them, but also because they don’t want to block the breeze. Adding screens to those homes would cost about $70 million, said Santana.

At dusk on a recent day, a maroon pickup truck drove through the streets in the middle-class neighborhood of Riveras de Cupey, in San Juan’s south, spraying permethrin, a commonly used insecticide, from a machine mounted on the back.

But Aedes aegypti mosquitoes already have developed resistance to permethrin in some parts of Puerto Rico, said Audrey Lenhart, a CDC research entomologist. She is testing which insecticides are most effective, something that was never done before.

How a bloodsucker transmits the Zika virus

“The Puerto Rican government doesn’t really have a well-developed vector control and surveillance program,” she said, referring to basic programs to eliminate insects, birds and other vectors that transmit disease.

CDC teams are helping authorities rebuild mosquito control programs, expand testing, and monitor and track thousands of pregnant women and their babies. They also are working with U.S. companies to provide window screens for women’s homes, and to bring to market a CDC-invented trap that could be a potent and cheap way to snare and kill adult mosquitoes.

For doctors such as Alberto De La Vega, an expert in high-risk pregnancies at the University Hospital in San Juan, Zika is one of many serious concerns. He worries that additional Zika testing will create huge demands on an already burdened health system.

“We’re having problems getting supplies, but we have to uphold U.S. standards,” he said. He has modern ultrasound equipment, but he pays out of his own pocket for the paper sheets that cover exam room beds.

He tells his patients they need to remove standing water and wear repellent.

“What we can do as physicians is very little,” he said. “By the time we identify problems with the fetus, it’s usually well into the second trimester, and by then it’s too late.”

Everything you ever wanted to know about the Zika virus and its spread across North and South America. (Daron Taylor,Claritza Jimenez/The Washington Post)
‘I’m going to have the baby’

The new mystery disease has infected Zulmarys Molina Paredes, 29. She’s one of the five pregnant women with a confirmed Zika diagnosis. But at 16 weeks in her pregnancy, an ultrasound shows her baby developing normally.

Molina and her 2-year-old son, Marco, live in Humacao in a peach-colored public housing project with her mother, aunt and brother. She is the sole breadwinner. She thinks she became infected at the private university where she works as an admissions officer, during tours of the campus. The campus has an artificial lake surrounded by trees full of mosquitoes.

Her headaches began Feb. 5. The following Monday, she looked in the mirror and was stunned.

“I was starting to put on my makeup and realized I was covered in a rash,” she said. “I got really scared.”

The emergency room doctor sent Molina’s blood to be tested. Nine days later, she was told her test was positive for Zika. But the doctor also said scientists didn’t know how often women with Zika infections have babies with birth defects such as microcephaly, where they are born with abnormally small heads.

Given the uncertainty, she is choosing to believe — and to pray — that everything will be fine. An amniocentesis is scheduled for next week. More ultrasounds will follow.

“I don’t care what happens. I’m going to have the baby,” Molina said. “I have faith that she’s going to be fine.” Her due date is Aug. 6. She will name her daughter Michaela.