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Local transmission of the Zika virus in Florida may have occurred as early as June 15 of last year.USA TODAY

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MIAMI — Women beware: if you're looking for a sperm donor, make sure you know where he's from.

The Centers for Disease Control and Prevention on Monday issued a warning that more people in South Florida's tri-county area — which includes Miami-Dade, Broward and Palm Beach counties — may have been exposed to the Zika virus than previously thought. And since the virus can remain in semen for up to three months, CDC officials said women who are pregnant or planning to get pregnant from donated sperm should consult with their doctors.

The officials said they have not recorded a single case of a woman or her fetus contracting Zika through donated sperm or through a blood transfusion - blood donations can be screened for Zika, sperm donations cannot. But the CDC officials said their continuing Zika studies found that more people throughout South Florida may have been exposed to the virus, since so many local residents cross county lines on a daily basis.

Denise Jamieson, incident commander for the CDC's Zika emergency response, said that means there could be plenty of men who made contributions to the area's 12 sperm banks who had no idea they were exposed to the virus.

"Some of the semen that was collected and stored may still be at risk for transmitting Zika," Jamieson said. "Some people in the area may not realize that they were at risk."

Zika became a nightmare for Miami last year when the city earned the dubious distinction of the first in the U.S. to experience active transmission of the virus, which can cause devastating birth defects in babies born to women infected while pregnant. The Florida Department of Health estimates that 279 people acquired the virus in the state in 2016, most of them in Miami-Dade County.

The virus, which originated in Brazil and is transmitted by the aedes aegypti mosquito, has also been detected in people in 49 U.S. states, mostly from Americans who contract the virus when traveling to countries where the virus is more prevalent. Florida leads the way with 1,095 travel-related cases, followed by New York with 1,009 cases and California with 431, according to CDC data. The U.S. territory of Puerto Rico has seen far more cases, with 36,967 people infected.

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The CDC had previously issued travel warnings for sections of Miami-Dade County where local transmissions were active. Those warnings were lifted as mosquito season died down in late 2016. The last confirmed case of locally-transmitted Zika occurred on Dec. 21.

But with winter coming to an end and mosquito season right around the corner, health officials are preparing to battle more Zika cases this summer. So many people living in the U.S. have already contracted the virus that former CDC Director Thomas Frieden warned last year that the virus was "not controllable" with current technologies.

On Monday, Jamieson gave a more optimistic assessment, saying that the virus is "not necessarily" endemic and that it could fade away, just as other mosquito-borne viruses have done in recent years.

"We're on the lookout for additional cases of Zika," she said. But, "if you look at dengue and chikungunya, (they give) some indication that we may expect small pockets of local transmission."