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Men who have traveled to Zika-affected zones should use a condom if they want to be absolutely sure they don’t infect sex partners, federal health officials advised Friday.

And men with a pregnant sex partner who have been to Zika-affected zones should just use a condom or abstain from sex until the baby is born, the Centers for Disease Control and Prevention said.

Health officials in Dallas said this week they had confirmed a case of sexual transmission in which one person traveled to Venezuela, where Zika is spreading, and come back to Dallas to infect a sex partner.

“Men with a pregnant sex partner who reside in or have traveled to an area of active Zika virus transmission and their pregnant sex partners should consistently and correctly use condoms during sex (vaginal, anal, or oral) or abstain from sexual activity for the duration of the pregnancy,” the CDC said in its latest advisory on Zika.

“Our priority here is to prevent a pregnant woman from becoming infected with Zika,” CDC chief Dr. Tom Frieden told reporters.

"The bottom line for most people in the U.S. is that pregnant women should postpone travel to Zika-affected areas. Our new guidance is that pregnant women should use condoms during sex or abstain if their partner has traveled to an area where Zika has been spreading."

“Our priority here is to prevent a pregnant woman from becoming infected with Zika."

Frieden said no one knows what would happen if a pregnant woman was exposed to Zika virus through sex, so it's not worth taking the chance.

It's known that Zika can get into a man's semen. It is not known how common this is, or how long the virus stays there. Frieden says CDC is working with scientists in Brazil and elsewhere to find out.

"These are studies that will take weeks and months," Frieden said.

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Every day, Frieden said, the links between Zika infection and a serious birth defect called microcephaly become stronger.

“The real problem here is the effect on the developing brain of the fetus,” Frieden said.

The CDC also updated its advice to women who are already pregnant and who have been or are in Zika-affected areas. Women with any symptoms of Zika, such as a rash or fever, should be tested for the virus right away. Pregnant women with no symptoms should get a test when they get their next prenatal visit.

Frieden said CDC adjusted its advice to women who don’t have symptoms of Zika infection, because it’s difficult to test them all and then to give them ultrasounds every few weeks.

A woman carries her grandson as a health official collects mosquitos and larva at her home to check for Zika virus at a village in Phnom Penh, Cambodia, on Feb. 4, 2016. SAMRANG PRING / Reuters

“Our labs are literally working around the clock to get more test kits available,” Frieden said.

“Although sexual transmission of Zika virus infection is possible, mosquito bites remain the primary way that Zika virus is transmitted. Because there currently is no vaccine or treatment for Zika virus, the best way to avoid Zika virus infection is to prevent mosquito bites,” CDC said in its advisory.

CDC issued a travel advisory last month telling pregnant women to stay away from countries where Zika is circulating.

It's still not 100 percent certain that Zika does cause microcephaly, Frieden said.

"Because this phenomenon is so new, we are, quite literally, discovering more and more about it each and every day," Frieden told reporters. The same is true for a rare but dangerous neurological syndrome called Guillain-Barre disease.

"And because it's new and because it can be so severe, it's scary," Frieden said.

Also on Friday, the World Health Organization said 26 countries in the Americas now have Zika circulating. "Seven countries have reported an increase in the incidence of cases of microcephaly and/or Guillain-Barré syndrome concomitantly with a Zika virus outbreak," WHO said. Brazil has the most Zika cases with more than a million, and Colombia has more than 20,000, WHO said.

It's not known at what stage in pregnancy Zika would be most dangerous, so any pregnant women should take care, Frieden said.

“Although sexual transmission of Zika virus infection is possible, mosquito bites remain the primary way that Zika virus is transmitted."

Women traveling to or living in Zika-affected areas who are not pregnant need to think about what they want to do, Frieden said.

"For women of reproductive age, healthcare providers should discuss strategies to prevent unintended pregnancy, including counseling on family planning and the correct and consistent use of effective contraceptive methods, in the context of the potential risks of Zika virus transmission," CDC advised.

Frieden repeated his belief that Zika will not spread explosively across the U.S., because the Aedes aegypti mosquitoes that transmit the virus are only found in very warm areas.

He said mosquito control is difficult, labor-intensive and expensive, but that all communities would benefit from doing as much as they can.

So far, Frieden said, CDC has heard about 51 cases of people coming back with Zika infections to the continental U.S. Six of these have been pregnant women and in one case a baby was born with microcephaly -- a woman in Hawaii who had been living in Brazil.

"No doubt many more travelers will return to the U.S. with Zika infections," Frieden said. "Some will be pregnant women."