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Florida now has a few cases of Zika. Puerto Rico has a widespread epidemic.

Aedes aegypti mosquito, the species which transmits the zika virus
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Brian Resnick is Vox’s science and health editor, and is the co-creator of Unexplainable, Vox's podcast about unanswered questions in science. Previously, Brian was a reporter at Vox and at National Journal.

Today, the Centers for Disease Control and Prevention announced that the Zika virus has begun to transmit locally in Florida. The mosquito-borne and sexually transmitted virus has, up to now, only entered the continental United States via travelers.

This announcement is based off four cases around Miami-Dade County. And as Vox’s Julia Belluz explains, it’s not a surprise. Health officials have for months been anticipating a small, local outbreak somewhere in the southeastern United States. Tom Frieden, the CDC’s director, predicted it in May: "We do expect there will be some spread — through mosquitos — in [these] parts of the continental US," he said.

But there’s a place in the United States where the Zika outbreak has already reached epidemic proportions: Puerto Rico. In the case of Puerto Rico, health officials are dealing with a major outbreak that is much more difficult to control.

CDC: The spread of Zika is “substantial and increasing” in Puerto Rico

As of July 7, some 5,582 people — including 672 pregnant women — have been diagnosed with the virus, the CDC said Friday in its latest Morbidity and Mortality Weekly Report.

And most worryingly, for pregnant women, diagnoses of suspected cases in Puerto Rico are up to 41 percent from 8 percent in February.

“This could lead to hundreds of infants being born with microcephaly or other birth defects in the coming year,” Lyle Peterson, a member of the CDC’s Zika Response team, said in a statement. “We must do all we can to protect pregnant women from Zika and to prepare to care for infants born with microcephaly.”

In June, the CDC confirmed one case of Zika-related microcephaly on the island. The report did not list any new birth defect cases, though it said the CDC and the Puerto Rico Department of Health would continue to monitor all women with confirmed or presumed cases of Zika.

The actual number of Zika infections is likely to be higher, as many carry the virus without showing symptoms.

The CDC also noticed a small association between Zika and Guillain-Barré syndrome — an autoimmune disorder that’s suspected to be trigged by Zika. Of 36 cases of Guillain-Barré in Puerto Rico, 21 were associated with Zika or a similar mosquito-borne disease.

What precautions is Puerto Rico taking?

The peak mosquito season in Puerto Rico stretches deep into September, and the CDC is clear that this outbreak is far from over. The agency is calling for increased mosquito control – insecticide spraying and the dissemination of bug repellent and bed nets.

Public education could also play a big role in curbing the outbreak.

“Since February 2016, approximately 21,000 pregnant women ... have been counseled about Zika virus prevention,” the CDC reports. “In addition, to reduce the risk for unintended pregnancies, the public health response includes community outreach and education about sexual transmission of Zika virus, distribution of male and female condoms, and an increase in the availability of the full range of voluntary contraceptive methods, including long acting reversible contraceptives.”

The CDC also reports that all public schools in Puerto Rico will be sprayed with Deltamethrin, an insecticide, to further prevent spread. (The Puerto Rican government has rejected aerial spraying for mosquitos, however.)

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