Who should avoid traveling because of Zika?

But Cordero, a former assistant U.S. surgeon general, and others say this is the wrong approach, given growing evidence men can transmit the virus to women sexually.

The mystery of Zika in semen

Uncertainties about Zika in semen underscore the disagreement about travel for men who hope to be fathers soon.

The virus is “spreading explosively” throughout the Americas, according to the World Health Organization, with millions of infections expected in the coming months. But no one knows whether all men who get infected end up with the virus in their semen.

And once the virus is there, no one’s sure how long it lasts. One report found that the virus persisted in a man’s semen for at least 62 days after he was sick with Zika.

A CDC official said while the agency is studying how long the virus lasts in semen, it continues to advise only pregnant women not to travel to Zika-affected areas.

“We want to learn as much as we can quickly to provide the best information we can to men and women thinking of starting a family,” said Dr. Anne Schuchat, principal deputy director for the CDC and the Agency for Toxic Substances and Disease Registry.

Schuchat said it’s best to let couples and their doctors make decisions about a man’s travel. “If you’re 22 and just thinking about having a baby at some point, that’s different than being 35 and really wanting to get going at it,” she said. “There are so many different circumstances around your fertility and your life plans. I think it’s really personal.”

Zika and condoms

Currently, the CDC is telling men who have traveled to Zika-affected areas to abstain from sex or use a condom with a pregnant partner.

If the partner isn’t pregnant, men “might consider” abstaining or using condoms, according to the guidelines.

That advice — as well as travel guidance for would-be fathers — could change as researchers learn more about how long Zika lasts in semen of infected men.

“It if turns out that 75% of men have it in their semen after recovery, we’ve really got to emphasize the condoms,” said Dr. Anthony Fauci, director of the National Institute of Allergies and Infectious Diseases.

“If it’s like Ebola, it could still be there nine months later,” he added.

For some experts, such uncertainty is the reason why the CDC should be warning men who want to become fathers soon not to travel.

“There’s been a fair amount of talk already that the CDC travel guidelines ought to be much firmer, much more prescriptive,” said Dr. William Schaffner, chairman of the department of preventive medicine at Vanderbilt University Medical School, who also serves on the CDC’s Advisory Committee on Immunization Practices.

Cordero agrees.

“Given that we know Zika can be transmitted through semen, if we’re telling women to postpone travel, we should be recommending something similar to men,” he said. “It only makes sense.”