Health officials in Dallas reported the first case of known sexual transmission of Zika in the current outbreak Tuesday.
Dallas County health officials said they had confirmed someone went to Venezuela, came back infected with Zika, and that person's sexual partner, who had not traveled, also became infected. Both people tested positive for the virus, the Centers for Disease Control and Prevention confirms.
"The patient was infected with the virus after having sexual contact with an ill individual who returned from a country where Zika virus is present," the county health department said in a statement.
Zika is usually carried by mosquitoes.
"We've confirmed that someone who didn't travel has had Zika in the U.S.," CDC director Dr. Thomas Frieden told NBC News. "And all of the information provided to us by Dallas makes it clear that it is likely sexual transmission."
Dallas authorities have given no details about the infected pair - whether it's a married couple, for instance. They have said there's no risk that either one is a pregnant woman.
"Everything we know about so far indicates that this is likely sexual transmission," Frieden said.
Zika's spreading fast across the Americas and the Caribbean and the World Health Organization has declared it a public health emergency of international concern. The virus itself is relatively harmless to most people, but what's worrying is the potential that it causes severe birth defects.
Brazil, where the virus has been spreading since last May, reports a large increase in cases of microcephaly - a condition in which a baby's brain is too small. It can cause miscarriages and stillbirths and babies that live are often severely disabled.
CDC issued a travel advisory last month, telling pregnant women, or women planning to get pregnant, that they may want to consider delaying travel to affected countries. A virus like Zika can spread very fast for a few years when an entire population of people is susceptible. After many people have been infected and are immune, then the spread becomes less intense.
The CDC has predicted that travelers would occasionally bring Zika to the U.S. but said any further spread would be very limited because the Aedes aegypti mosquito that carries the virus is restricted to warm areas in the extreme south — south Texas and south Florida, for instance — and Hawaii.
There may be local outbreaks, CDC says, but not the explosive spread that's now got the virus infecting people in 26 countries.
Doctors had known Zika could be spread by sexual transmission. In 2008 a U.S. researcher was infected in Africa and infected his wife back in Colorado. Zika has been found in semen.
"This is no surprise," Frieden said. "We have seen cases or isolated cases of Zika spread by sexual transmission." But he says the virus is not likely to linger for years or even months, because Zika is a virus the body can clear out. "It's not like HIV where you have infectivity that can go on for months or years," Frieden said.
"The bigger concern is if a man comes back, gets sick with Zika, and then has sexual intercourse with a woman who is pregnant or may be pregnant, there is the theoretical risk that that woman could then become infected with Zika and her fetus could then potentially be infected," Frieden added.
"Now that we know Zika virus can be transmitted through sex, this increases our awareness campaign in educating the public about protecting themselves and others," said Zachary Thompson, director of Dallas County Health and Human Services.
He said sex partners of people who could potentially be infected with Zika should consider using condoms for a week after their return from affected areas. "Next to abstinence, condoms are the best prevention method against any sexually transmitted infections," Thompson said.
Georgetown University infectious disease specialist Dr. Jesse Goodman agreed.
"If confirmed, this case suggests the potential to further guard against risk to pregnant women, or women intending to become pregnant, with advice to reduce risks from sexual contact if their partner may have recently been exposed to Zika from mosquitos in countries where there is ongoing active transmission," Goodman said.
"Because of rapidly changing knowledge, advice about protecting against Zika may well change over time as more is learned."
Dallas officials have given few details. They did not disclose the sexes of the couple and they did not say if a man's semen had been tested for the virus. They also did not say if the traveler had shown symptoms of infection. Zika does not cause symptoms in most people it infects and in about 20 percent causes a mild rash, fever, muscle aches and pinkeye.
So it's not known if people have to show symptoms in order to transmit the virus.
"Everything we know about Zika suggests that the overwhelming majority of cases are spread by mosquitoes," Frieden said.
"In this individual case the Dallas health department is investigating and they would have to respond to initial questions. But basically from everything we've seen it does seem likely that that was sexual transmission and not that it was spread by a local mosquito."
"This highlights our ignorance of this virus and the need for an urgent, comprehensive and coordinated research response," said Peter Horby, a professor of emerging infectious diseases and global health at Britain's University of Oxford.