US health officials Friday reported the first known case of a man who acquired Zika virus while traveling, showed no symptoms, and infected his female partner during unprotected sex.

The case suggests the risk of Zika's spread may be far greater than previously understood, and may lead to more stringent recommendations about who should practice protected sex and for how long after traveling to Zika-affected areas, said the US Centers for Disease Control and Prevention.

The man, who lived in Maryland, traveled to the Dominican Republic earlier this year and returned without reporting any symptoms of Zika, which can include rash, joint pain, fever and red eyes.

He had condomless sex with a female partner on two occasions, 10 and 14 days after his return.

On the 16th day after his return, the woman developed a fever and rash.

In June, Maryland health authorities were notified of the case, and the woman tested positive for Zika.

She reported no other sex partners in the two weeks before her infection arose, and no blood transfusions or organ transplants either.

The man got tested 29 days after his return, and the results confirmed "a recent, unspecified flavivirus infection," said the CDC, referring to a family of viruses which includes Zika, dengue and West Nile.

"Semen collected on Day 31 had no detectable Zika virus."

However, the man said his only symptom was feeling a bit tired, which he chalked up to having recently traveled.

Zika is primarily a mosquito-borne virus that causes no symptoms in four out of five of those affected.

But if pregnant women are infected, they face a higher risk of having a baby with head and brain defects, a condition known as microcephaly.

In this case, the female partner was not pregnant, the CDC said.

Currently, the CDC urges couples who want to become pregnant to wait at least eight weeks if one of the partners has traveled to an area with active Zika virus transmission but did not develop symptoms.

But now, the "findings in this report indicate that it might be appropriate to consider" anyone who has unprotected sex with a person returning from an area with ongoing Zika virus transmission "as exposed to Zika virus, regardless of whether the returning traveler reports symptoms of Zika virus infection," it said.

Men who are diagnosed with Zika should wait far longer – at least six months – before attempting conception, officials said.

And women who are confirmed positive for Zika are urged to wait at least eight weeks before trying to get pregnant.

Researchers are aware of just one other case in which a man without symptoms might have sexually transmitted Zika virus to his female partner.

But in that case, since they both had traveled to a country with ongoing Zika transmission, it could not be ruled out that the woman was infected by a mosquito bite.