Dallas health officials said Tuesday that a local resident was infected with the Zika virus by having sex with a person who had contracted the disease while traveling in Venezuela.

The case adds a troubling new dimension to a once-obscure disease that is spreading explosively in the Americas and is suspected of being linked to hundreds and perhaps thousands of cases of babies with brain damage in Brazil.

Dozens of other Zika cases have surfaced in the United States, but they have involved people who became infected with the mosquito-borne virus while traveling to Zika-affected countries. Tuesday’s case marks the first report in the current outbreak of someone being infected without leaving the United States, health officials said.

The development prompted the Centers for Disease Control and Prevention to quickly issue new recommendations urging the use of condoms to prevent the spread of the virus.

Also Tuesday, the American Red Cross appealed to prospective donors who have visited Zika outbreak zones to wait at least 28 days before giving blood, but said risks of transmitting the virus through blood donations remained “extremely” low in the continental United States, Reuters reported.

Dallas health officials said Tuesday they had been tracking the local case for weeks. They declined to disclose details about the two people involved, but said the incident began with a traveler who likely had gotten infected through a mosquito bite while in Venezeula. That person, after returning to Dallas County, had sex with the second person, who soon began having flu-like symptoms and went to the doctor.

Christoper Perkins, the medical director for Dallas County, said the doctor recommended that the patient be screened for Zika after learning the person had not traveled outside the area and had gotten sick after having sex with the traveler.

Perkins also said mosquitoes are still rare in the area at this time of year, making it even more likely the disease was transmitted through sexual contact.

Officials sent blood samples to the CDC about two weeks ago, Perkins said. The CDC ruled out other possible viruses, including dengue and chikungunya, eventually confirming the Zika diagnosis in both individuals.

Dallas County Judge Clay Jenkins said in an interview late Tuesday that authorities tested mosquitoes caught near the couple’s home and did not find the species that is known to transmit the virus.

In this instance there was no risk to a developing fetus,” the CDC statement said.

Public health officials have stressed that the Zika virus, which has now spread to nearly 30 countries and regions, is transmitted primarily by mosquitoes. There have been isolated reports of transmission through sexual activity. But if the virus is shown to be transmitted readily through sexual contact, it could only further complicate efforts to halt Zika’s spread.

The CDC is now saying that the best way to avoid Zika virus infection is to prevent mosquito bites and for sexual partners to wear condoms to prevent sexually transmitted infections.

“We do not have definitive information on the infectious time period, and will provide more guidance for individuals and clinicians as we learn more,” the agency said.

CDC said it will issue guidance in the coming days on prevention of sexual transmission of Zika virus, with a focus on the male sexual partners of women who are or who may be pregnant.

Until more is known, the agency said, the CDC continues to recommend that women and women trying to become pregnant postpone travel to the areas where Zika virus transmission is ongoing. Pregnant women who live in or travel to one of these areas should talk to their doctor or other health-care professional first and strictly follow steps to avoid mosquito bites during the trip.

Pregnant women should also avoid exposure to semen from someone who has been exposed to Zika virus, the agency said. Women trying to become pregnant should consult with their health-care professional if their partner has had exposure to Zika virus.

By contrast, British authorities advised last week that men use condoms for at least 28 days after returning from Zika-affected areas. Public Health England said men who had fever, rash and joint pain should avoid having unprotected sex for six months.

Health experts said the Dallas case raises new questions about how the virus is spread.

“It looks like a pretty well documented case of sexual transmission,” said Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.

“Obviously, this adds a new dimension that needs to be addressed,” he said, noting CDC’s plans to update its guidance on the subject.

Still, he agreed with other researchers that many unanswered questions remain when it comes to how Zika is transmitted sexually. “How long is it contained in the semen? Is it a day or a week or a month?” he said. “Or is it like Ebola, which we know can remain in semen for several months? … We just don’t know.”

“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, Dallas County’s health director, in a statement. “Next to abstinence, condoms are the best prevention method against any sexually transmitted infections.”

Researchers have been speculating about the possibility of transmission of Zika virus through sexual contact since 2008, when an American scientist who had been traveling in southeastern Senegal was suspected of transmitting it to his wife.

In the days after his return to Colorado, the man and his wife had sex. Several days after that, the man became symptomatic, experiencing swollen ankles, a rash on his torso and extreme fatigue and headache, but no fever, according to a description published in the journal Emerging Infectious Diseases. His wife – who had not traveled out of the country in the previous year – experienced similar symptoms.

The man and his wife’s infections with Zika were confirmed through blood tests, but the researchers did not investigate whether the virus was present in his semen. Researchers speculated at the time that infected semen may have transmitted the virus, or that another bodily fluid, such as saliva, could be responsible. They did note that the illness did not develop in the couple’s four children, however.

Brian Foy, an associate professor at Colorado State University who works on infectious diseases and was the lead author of the paper, subsequently told journalists he was the scientist being discussed.

The issue came up again in 2013 when doctors in French Polynesia isolated a high concentration of actively replicating virus in a 44-year-old man’s semen and concluded that “this observation supports the possibility that [Zika virus] could be transmitted sexually.”

The World Health Organization designated the Zika virus and its suspected complications in newborns as a public health emergency of international concern Monday. The action, which the international body has taken only three times before, paves the way for the mobilization of more funding and manpower to fight the mosquito-borne pathogen spreading “explosively” through the Americas.

The case marks the second time in recent years that Dallas has found itself thrust into the spotlight of an international infectious disease outbreak.

In September 2014, a Liberian man named Thomas Eric Duncan became the first person diagnosed with Ebola in the United States after he traveled to Dallas to visit family members. His diagnosis came shortly after he traveled from his home country, bringing the deadly disease with him across the Atlantic, where an unprecedented Ebola outbreak was unfolding.

A Dallas hospital initially released Duncan, but he was readmitted shortly thereafter. He died in earlier October. Two nurses who tended to him at the hospital soon tested positive for the virus but later recovered. The incident also sent health officials scrambling to identify everyone who might have come into contact with him since he began showing symptoms of the disease.