In a cramped cinder-block apartment in a slum blighted by warring gangs, Stefanie Ramirez listened as a doctor rubbed her pregnant belly and spoke gently about what was happening to her unborn child — and to her country.

Ramirez contracted the Zika virus painlessly, experiencing no fever, only a minor stomach rash. She recovered, returned to work as an accountant and hardly thought about the sickness until the past few weeks, when the possibility of a link between Zika and babies born with abnormally small heads grew so alarming that Salvadoran authorities advised women to avoid getting pregnant for two years.

Since then, Ramirez has been among 122 pregnant women across El Salvador who receive regular doctors’ visits and home sonograms to detect early signs of microcephaly. The monitoring is part of a nationwide effort to combat an illness that is rapidly spreading across the Americas.

“It’s an epidemic disease with serious consequences, especially for pregnant women,” Vladi­mir Ruiz, one of the doctors, told Ramirez during his visit Thursday.

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

This small Central American nation, and Ramirez’s neighborhood of San Jacinto in particular, demonstrates why the mosquito-borne disease marching north toward the United States is going to be so difficult to stop. In this web of slums, there are blocks where 8 in 10 houses are breeding sites for mosquitoes. The city is a patchwork of rival gang territories that are defended so fiercely that health authorities cannot enter some neighborhoods. In just the first three weeks of January, El Salvador recorded 2,474 new suspected Zika cases, nearly half of them here in the capital. Many infected pregnant women live in these densely packed southern neighborhoods.

“It’s uncontrollable,” said Eli Leiva, 40, an elementary school teacher in San Jacinto who has several students with Zika. “It’s a problem that has gotten totally out of hand.”

Doctors are worried that basic public-health messages are not reaching their audience. Many residents ignore the recommendation to destroy mosquito breeding grounds by disposing of standing water, even though El Salvador has suffered repeated outbreaks of dengue and chikungunya, fevers transmitted by the same type of mosquito that carries the Zika virus. Teen pregnancy is rampant, abortion is illegal and contraception is discouraged in the heavily Catholic country. Many women interviewed dismissed the advice not to become pregnant as unrealistic.

“People aren’t going to change because the government tells them to,” said Jennifer Estefany, 20, a mother of two who was visiting a clinic with her sick 4-year-old son. “The majority of people think this is some kind of lie.”

To fight the disease, Salvadoran authorities have launched a campaign to fumigate some 55,000 houses each week. Clinic workers hand out free packets of disinfectant for water supplies, and they lecture patients on mosquito prevention. Authorities are even giving people baby tilapia, hoping the fish will eat mosquito eggs and larvae living in water tanks.

“It’s easier to eliminate breeding grounds of mosquitoes than to limit people’s desire to be a mother or a father,” said Julio Morales, director of the Unicentro hospital in Soyapango, another southern neighborhood in the capital. “Killing the larva is fundamental. If people don’t understand that, we’re never going to stop this virus.”

Up and down the jungly hills of San Jacinto on Thursday, dozens of municipal employees hunted their prey using flamethrower-style fumigators with names such as Super Hawk and Vector Fog. On block after block, they came across abandoned or locked homes — and residents wary of the government. “We ask, at the very least, that you open your doors to the health people,” one doctor said over a megaphone as he drove through the neighborhood.

“Could you let us enter, please?” environmental inspector William Cabrera asked outside the metal gate of one house.

Ducking under a hammock, he followed Maria Magdalena Palomo de Hernandez, 76, into the house and headed for a back patio crammed with hanging laundry and tropical plants.

“Containers of water?” Cabrera asked.

He peered into a blue plastic barrel, half-full and partially covered. Across poor neighborhoods of San Salvador, including San Jacinto, the municipal supply of fresh water is sporadic. Faucets sometimes are dry for days or weeks, so residents store fresh water in buckets and barrels, ideal for the Zika-carrying Aedes mosquitoes, which prefer to lay their eggs in clean, stagnant water.

“Larvas!” Cabrera shouted, pointing. “See how many there are?”

“We’ve been trying to kill them,” Palomo de Hernandez said quietly.

“It’s a lack of education,” Cabrera said. “The problem is we’ve become so irresponsible we think the government has to solve all our problems. But people don’t want to collaborate and be part of the solution. That’s why, as a country, we’re not moving forward.”

Danger of gangs

The barrio of San Jacinto sits in the southeastern hills of the capital, tucked into a bend in the Acelhuate River. Once a thriving colonial town with ornate mansions and churches dating to the 17th century, the community was eventually engulfed by the expanding metropolis.

The main rival gangs that plague most of the country are also entrenched here. Residents are so afraid of them that many refer to the 18th Street gang only as “the numbers” and its enemy, Mara Salvatrucha, “the letters.”

A truce between the gangs collapsed two years ago. Since then, the number of homicides has skyrocketed, making El Salvador one of the deadliest countries in the world. Last year, the country of 6 million recorded 6,600 homicides, a 70 percent increase from 2014. Thousands of residents have fled north through Mexico to the United States, abandoning their homes. Health workers say they cannot enter some neighborhoods.

“About 40 percent of the area is difficult, though not impossible, to get to,” said Morales, the Unicentro hospital director, who estimates that he is responsible for some 150,000 people.

Teenagers, even children, will tag along behind health workers on their rounds and report back to gang leaders, said Claudia Muñoz, 46, a municipal employee who was participating in the fumigation in San Jacinto. “They think we are carrying information to the other gang. They think we’re spies.”

After two gang gunmen robbed the nearby San Cristobal clinic in 2013, the staff now makes house calls in a group, refusing to walk outside alone. Medical care involves delicate negotiations, including phone calls from imprisoned gang leaders. The gangs have agreed on occasion to do their own fumigation, using Health Ministry equipment.

Health inspector Salvador Quintanilla has been assaulted three times in his 25 years in the job. His list of no-go neighborhoods “isn’t a list anymore,” he said. “It’s practically a folder.”

No microcephaly yet

The link between the Zika virus and microcephaly, a correlation discovered in Brazil, where some 4,000 babies have been born with the condition, has not yet been confirmed in El Salvador. The virus was discovered here in late November, when health authorities sent 10 samples of an unknown virus to a U.S. Centers for Disease Control and Prevention laboratory in Fort Collins, Col. Three of the samples tested positive for Zika, according to the deputy health minister, Eduardo Espinoza. El Salvador still does not have the capacity to definitively identify the virus, so the mounting numbers here are considered “suspected” cases.

“The epidemic is on the rise,” Espinoza said in an interview.

Behind the barred windows of apartment 3-A in the San Jacinto housing project where Stefanie Ramirez was being examined, the seriousness of the epidemic was being gauged by the tiny contours of her unborn daughter’s skull. After administering the sonogram, Ruiz told her that her child, expected on Valentine’s Day, so far appeared normal. “We don’t see fetal malformations at the moment,” he said.

Ramirez told him she had heard on the news that there were no confirmed microcephaly cases in El Salvador.

“No,” Ruiz agreed. “Not yet.”

Read more:

Zika virus FAQ: What is it, and what are the risks as it spreads?

As Zika virus spreads, El Salvador asks women not to get pregnant until 2018

Why the United States is so vulnerable to the alarming spread of Zika virus

Today’s coverage from Post correspondents around the world