SAN JUAN, Puerto Rico — As the Zika virus speeds through the hemisphere, a cluster of one-story buildings set in the shade of banyan and mango trees has become the epicenter of the U.S. fight against the pathogen.

Behind an unmarked gated entrance in this working-class neighborhood of San Juan, scientists from the Centers for Disease Control and Prevention have only one focus: to stop Zika's spread. About 100 virologists, biologists, entomologists and more are working here, including dozens brought from CDC sites in Colorado and Georgia. They're racing to develop tests for faster and more accurate diagnosis of infection and killing Aedes aegypti mosquitoes by the hundreds to determine what chemicals are most effective.

Much of it is labor-intensive. The insecticide research means collecting eggs on sticky brown paper placed all over the island, then soaking those egg-dotted sheets in shallow trays of water back at the lab. The eggs grow from larvae into adult mosquitoes, which are kept in mesh cages and fed pig's blood contained in what looks like a membrane-wrapped sausage. The final stage? Putting the adults inside chemical-coated bottles. If 98 percent are dead within 30 minutes, the chemical has real potential against Zika's prime source of transmission.

The effort is critical, according to entomologist Audrey Lenhart, because much of the mosquito population has become resistant to permethrin, one of the most commonly used insecticides in Puerto Rico and abroad.

“It’s very impressive," CDC Director Tom Frieden said after visiting the complex during a recent trip. "You see 20 mosquitoes all flying around happily in a bottle that’s been coated with an insecticide that is being widely used and next to it another bottle where all 20 mosquitoes have been rapidly knocked down [by the experimental substance] and most of them killed."

In the midst of their research, the scientists also are scrambling to protect the population most at risk here. The island already has at least 249 confirmed Zika cases, almost as many as the rest of the entire United States. Twenty-four of those involve pregnant women, and the number could skyrocket given the 4,000 pregnant women who live in areas with active transmission.

Not only is the CDC helping the authorities install window screens on homes in these communities, it is working with local authorities to distribute 4,000 Zika prevention kits. The contents include condoms as well as repellent because sexual transmission is now proven.

Until January, the lab's target was dengue fever — the reason this branch was set up 30 years ago. Dengue fever is one of the world's most virulent infectious diseases, with more than 400 million people becoming infected each year. But today, all of the branch's work is shifted to Zika.

"There is no silver bullet to control the Aedes aegypti mosquito or reduce the risk of Zika infection on a population-wide basis, but there are some things that we may be able to do if we have the resources that would significantly reduce risk," Frieden said.

Conditions in Puerto Rico provide scientists a unique opportunity to study the virus and its effects in real time, noted Tyler Sharp, acting head of the epidemiological team. Teams are monitoring the pregnant women with the rash, red eyes and joint pain that are signs of Zika and visiting the homes of pregnant women without symptoms. They hope to learn more about Zika's link to a range of birth defects, including microcephaly, in which children are born with abnormally small heads and underdeveloped brains.

Researchers expect to see an uptick in Guillain-Barré syndrome, a rare neurological disorder that may be triggered by Zika. So far, there is only one case. But epidemiologists are scouring three hospitals' patient records going back to 2012 to establish the syndrome's prevalence before the Zika outbreak. With a baseline, Sharp said, they'll know the point at which additional cases represent a true increase.

The CDC wants to address another key question, too: "How long is Zika detectable and infectious in blood, semen, saliva, and vaginal secretions?" Sharp said.

What is learned in Puerto Rico should benefit the several dozen countries and territories in the Americas that are battling Zika. Cuba is the latest, as signaled Saturday when the CDC added it to the agency's travel-advisory list of places with active Zika transmission.

One of the biggest challenges in halting the spread of the virus is figuring out if someone is infected. It’s complicated because 80 percent of people don’t show symptoms. More than a year ago, lab scientist Gilberto Santiago helped to develop a new diagnostic that simultaneously tests for Zika, dengue fever and chikungunya, related viruses that tend to spread in the same regions. Known as the trioplex, it has been used in Puerto Rico since January. On Thursday, the Food and Drug Administration gave emergency approval to the three-in-one test, and on Monday, the CDC plans to start shipping hundreds of kits to labs in the United States and other countries.

“This will help the public in the most affected countries,” said Jorge Munoz, CDC director of diagnostics and research.

Another major hurdle is eliminating mosquitoes outside homes without using insecticides. Some of the lab's past research, led by entomologist Roberto Barrera, suggests one answer.

Barrera's team designed a low-tech trap — a 5-gallon black plastic bucket filled with water and hay. On top is a small, round chamber lined with special adhesive paper. Female mosquitoes looking for containers of water to lay their eggs are attracted by the odor of the hay decomposing, Barrera said. They fly into the chamber, get stuck on the paper and die within minutes.

Versions of the traps, which Barrera described as cheap, durable and low-maintenance, have been tested in southern Puerto Rico for the last several years. Communities with them had 80 percent fewer mosquitoes than communities with no traps, he said.

The researchers now are talking with companies about potential mass production.

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