Meet Zika, the latest virus that turns out to be a sexually transmitted disease. The mosquito-borne pathogen is getting a little extra help from humans who are passing it among themselves via sexual contact. And it is happening a lot more often than scientists previously knew.

Earlier this week the World Health Organization announced that transmission of the virus via sexual contact is “more common than we thought.” The U.S. is now feeling that reality firsthand. Yesterday, officials from Florida, the state that has amassed the highest number of travel-related cases of Zika, announced its first case of virus transmission via sexual contact.

Officials from New Hampshire announced their own case of sexually transmitted Zika last week and Texas and Colorado have also had similar occurrences. “Documentation of sexual transmission being possible is now proven,” says Tom Frieden, director of the U.S. Centers for Disease Control and Prevention. In each instance, a male who traveled to an area with active transmission of the virus returned to the U.S. and passed the virus on to a female who had not traveled—making it easier to tease apart how the woman contracted the virus in the first place.

But difficulties understanding how common this mode of transmission is in areas where people also encounter Zika-carrying mosquitoes continue to hinder any efforts to get a better sense of sexual transmission occurrences worldwide. Brian Foy, a Colorado State University researcher who unwittingly transmitted the virus to his wife via sexual contact in 2008, is planning to lead one effort to try to get some answers in Brazil. In a study that is still awaiting federal funding his team plans to administer testing and sexual history questionnaires to both patients in the acute throes of Zika virus and to a control group of patients that instead have chikungunya or dengue—hoping to glean some overarching indications about Zika via sexual transmission. But the study will be far from definitive.

“The idea is to conduct this study during the time of year with low levels of mosquito-mediated infection, in the Southern Hemisphere winter,” and to ask these patients about their sexual histories, says Ernesto Marques, an infectious diseases public health researcher at the University of Pittsburgh who is working with Foy. “But I can’t rule out [that these infections are from mosquito bites],” he adds.

Getting answers about the incidence of sexually transmitted Zika is essential. “If we spend a lot of money on mosquito vectors and find out most transmission is sexual, then it won’t do anything,” Marques says. “The other issue is we don’t know if the form of transmission of the virus has an implication for congenital disease. Perhaps sexual transmission is more likely to induce or result in congenital disease,” he says. Right now there is no evidence to support or refute that idea but it is important to get that information, he says.

Already there is a growing body of literature linking Zika virus with serious birth defects, among them microcephaly. A study published in The New England Journal of Medicine last week found that 12 of 42 pregnant Brazilian women who tested positive for Zika—29 percent—had fetal abnormalities that were apparent via ultrasound.

Anthony Fauci, director of the U.S. National Institute for Allergy and Infectious Diseases, says that the finding is particularly concerning because there will likely be many more birth defects that are not apparent until after birth. It is also “alarming,” he says, that the study found there were issues among women who contracted Zika later in pregnancy, too—not just the first trimester—suggesting there may be Zika-linked problems no matter when the initial infection takes place.

The more we learn about Zika virus “the worse things seem to get,” Fauci says. Ongoing studies suggest that, at least in some areas, the mosquitoes carrying Zika have significant resistance to common insecticides. And Zika’s links with the autoimmune disease Guillain–Barré syndrome are only getting stronger.

Work published last month in The Lancet documenting links between Guillain–Barré and an earlier Zika outbreak in French Polynesia is “highly suggestive,” according to Frieden, but it is not yet definitive. By the end of the month his agency will be publishing work replicating those findings in partnership with Brazilian public health authorities, he says.

For now, the CDC is trying to help control the virus in Puerto Rico where it is already actively spreading via mosquito bite. Ahead of the U.S. territory’s rainy months—and likely Zika explosion—the agency is handing out Zika prevention kits there that include condoms, insecticides and information for women about how to protect themselves against the virus.