The World Health Organization on Tuesday expanded its guidance on preventing Zika, warning that everyone who visits a Zika-infected zone should practice safe sex for six months, even if they show no symptoms of the virus.

The new recommendation applied to men and women, a significant change in the United Nations agency's view of the dangers of sexual transmission. Previously, only men with symptoms of the disease were told to practice safe sex for six months. For women, the period was eight weeks.

The U.S. Centers for Disease Control and Prevention said that it was considering updating its own guidance on safe-sex practices in light of the WHO recommendation.

The CDC currently recommends that men and women returning from areas where Zika is prevalent abstain from sex or use condoms and barriers for eight weeks if they show no symptoms. Because of the time Zika is thought to remain in semen, men who show symptoms should wait at least six months before having unprotected sex, the CDC has cautioned. Women are advised to wait eight weeks after first showing symptoms.

But WHO officials said that is no longer long enough.

The new guidelines reflect growing concern among health officials about the role of sex in spreading the virus, which can have a catastrophic effect on fetuses, including a brain disorder characterized by microcephaly.

Sexual transmission had initially been thought to be relatively rare. But the spread of the virus worldwide — including into northern areas where the primary source of the disease, the Aedes aegypti mosquito, is not prevalent — suggests a far greater role for sex in Zika transmission.

"Mounting evidence has shown that sexual transmission of Zika virus is possible and more common than previously assumed," the WHO said in its new guidelines.

While experts agree that mosquitoes remain the main concern, recent studies indicate that women have been infected with the virus more often than men have, possibly because of sexual transmission. Researchers also think that women can transmit the virus to their partners.

Eleven countries have reported cases of sexually transmitted Zika, according to the WHO. CDC officials know of 23 sexually transmitted cases in the continental United States.