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he Centers for Disease Control and Prevention on Friday issued new guidance aimed at reducing the risk of sexual transmission of Zika, the mosquito-borne virus that has spread across the Americas and been tied to an alarming rise in birth defects.

The advice is squarely aimed at preventing women from becoming infected during pregnancy. “The real problem here is the effect on the developing brain of the fetus. That is what has to be the priority for protection,” said CDC Director Dr. Thomas Frieden.

Sexual transmission of Zika is believed to be rare. Still, the CDC said, men with a pregnant sex partner who live in or have traveled to an area where Zika is spreading should use condoms during sex for the duration of the pregnancy or abstain from sex altogether. In these cases, the CDC recommended the use of condoms for vaginal, anal, and oral sex.

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Outside of pregnancy the advice is less prescriptive. The CDC said that couples may consider using condoms or abstaining from sex if a male partner lives in or has traveled to a place where Zika virus is spreading.

The guidance is based on the assumption that only infected men can transmit the virus sexually; to date there is no evidence infected women can pass the virus to partners through sex.

The conditions under which Zika can be transmitted sexually are not well-understood. Scientists are still trying to determine, for instance, how long the virus remains in the semen of a man who has recovered from Zika.

Public health officials in Dallas earlier this week reported the second known US case of sexually transmitted Zika. The only previous case was in 2008.

The new CDC advice does not discuss kissing. On Friday, Brazilian scientists announced they had found active virus in saliva and urine samples from people infected with Zika.

When asked about the report, Frieden said he would need to see details of the work before commenting.

The CDC’s counterpart in England, Public Health England, has recommended that women whose partners have traveled to a Zika-affected area avoid getting pregnant for 28 days after a male partner’s return, assuming he is not showing symptoms of the disease. If he is showing symptoms, Public Health England says, his partner should avoid pregnancy for six months after his recovery.

The issue is that many people who contract Zika virus might not know they have it. Only 1 in 5 people who have Zika develop symptoms, and those symptoms are typically mild: fatigue, aches, pain, and a skin rash. Illness generally lasts several days to a week.

Scientists have theorized that Zika is causing a spike in the number of Brazil’s cases of microcephaly, a condition in which newborns have abnormally small heads and underdeveloped brains.

This story has been updated to include comments from CDC Director Dr. Thomas Frieden.

Are you at risk for contracting Zika virus? Your level of risk depends in part on your living conditions. Alex Hogan/STAT

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