Fourteen Miami-area residents who have caught Zika virus in the city are the first believed to have been infected by U.S. mosquitoes. The news confirms what public health experts have been predicting for months: At some point this summer in the United States, Zika would cease to be only an imported illness and become a locally transmitted one.

But reaction to the Florida discoveries—four cases announced Friday and ten today—obscures another development. The Centers for Disease Control and Prevention has confirmed something that’s been implicit in discussions of the virus for a while: Zika is effectively a sexually transmitted disease.

Public health experts now know that Zika can be passed in bodily fluids between one man and another, between a man and a woman, and from a woman to a man—and though no case has been made public, they assume it can be transmitted between female partners as well.

So far, in the continental United States, 15 cases of Zika are confirmed to have been transmitted by sexual contact. That’s out of 1,657 cases of infection. (In Puerto Rico, where Zika is spreading very rapidly—4,684 cases as of July 29—analysis for sexual transmission hasn’t been done.)

The CDC’s updated advice—a result of the discovery two weeks ago that a woman in New York City passed Zika to a man she had sex with—expands advice the agency previously gave about protecting pregnant women. Now it says: “Men and women who want to reduce the risk for sexual transmission of Zika virus should use barrier methods against infection consistently and correctly during sex or abstain from sex when one sex partner has traveled to or lives in an area with active Zika virus transmission.”

The point of the new advice is twofold: to protect pregnant women, or ones about to become pregnant, from risking a devastating birth defect; and also to slow down the transmission of Zika from infected travelers into the rest of the population. But it emphasizes the tricky nature of detecting and preventing the advance of Zika, since four out of five people infected show no symptoms.

“We have been a little bit frustrated by the lack of focus on Zika as an STD,” William Smith, executive director of the National Coalition of STD Directors, said by phone, “when there has been credible evidence that sexual transmission could be as important a focus for us in preventing Zika as mosquito-borne transmission.”

CDC-sponsored signs at airports, he pointed out, talk about avoiding bites and wearing mosquito repellent, but don’t show condoms. “We have a great tool to prevent this,” he said.

John Brooks, a physician who’s leading the sexual-transmission section of the CDC’s Zika response, said by phone that the agency had been expecting to add sexual transmission of Zika to the ways the disease can be passed. It has been reported in ten other countries, he said, and in the United States, the known first case of sexual transmission (in a tropical-disease researcher returning home) was in 2008.

Until the giant South American outbreak began, Zika was under-researched, and that was true for sexual transmission of the disease as well, Brooks said. But based on those, it seems clear that Zika virus can persist in some bodily fluids longer than it does in the bloodstream, which is where mosquitoes pick it up. Some small studies have shown viral persistence in urine and saliva up to 21 days, and a recent study revealed the virus lingers in cervical mucus as well.

The biggest risk appears to be semen: Studies have shown molecular traces of the virus, though not necessarily infectiousness, for as long as 93 days. But the science that could determine how long that transmission risk lasts, and whether it is the same for people with no symptoms, has not yet been done.

“We are launching some big studies to try to understand persistence in bodily fluids,” Brooks said.

The season for the mosquito species that transmit Zika has just begun, but there’s no “season” for sex. The CDC estimates that 40 million people travel to the United States in a year from the Zika-affected zones. Doing some back-of-envelope calculations—only a certain percentage of travelers will be infected, only a certain percentage of those will have symptoms, only some of those will have sex while in the U.S.—Brooks said that, if sexual transmission were as important in spreading the virus as mosquitoes, the CDC believes more cases should have shown up by now.

On the other hand, that would require an alert doctor or nurse to notice the symptoms of fever, rash, and red eyes and ask questions—which doesn’t always happen. “I think we have primed the clinical community and travelers in the United States pretty well by now,” Brooks told me. “If somebody showed up with a rash and conjunctivitis and a low grade fever, somebody would begin to probe.”

But uncovering the sexual link also requires the health-care professional to ask additional questions—which, even after decades of the AIDS epidemic, turns out to be surprisingly hard. “We clinicians are horrible at taking sexual histories and travel histories,” Brooks agreed. “But that is one of our jobs here at the CDC, to raise awareness about this.”

It was alert health-care workers who made the connection in Florida between Zika and the first four locally infected people. The Florida Department of Health says the new cases were “likely contracted” through the bites of local mosquitoes. (Before Friday’s announcement, 386 Florida residents had been infected with Zika by traveling to places where the disease is circulating, including 55 pregnant women.)

“Florida has become the first state in the nation to have local transmission of the Zika virus,” Florida Gov. Rick Scott said in Friday’s press conference. Today, the Florida health department said all of the people infected are linked through a neighborhood called Wynwood, an entertainment district just north of downtown Miami.

Though the health department said no local mosquitoes had yet been found carrying the virus, the state said it would immediately begin spraying for mosquitoes and also perform urine testing of people living and working there to see if others are carrying the virus. Separately, the Food and Drug Administration asked blood banks in Miami-Dade and Broward Counties to stop collecting blood until they could begin testing it for the presence of Zika virus.