U.S. health experts cautioned Friday that the apparent discovery of the Zika virus in saliva and urine from people in Brazil does not necessarily mean the virus can be spread by more casual contact with infected people, such as through kissing.

"I think we need to be careful that don't we jump to any conclusions about transmissibility," Anthony Fauci, who leads the National Institute of Allergy and Infectious Diseases, said during an interview on NPR's Morning Edition.

"When you find a virus or fingerprints of a virus in a body secretion, it absolutely does not mean that it is transmitted that way," Fauci said.

The virus may be present in a form that is incapable of spreading, or in such low levels that transmission is impossible or unlikely, several scientists said. There also could be substances present in saliva that prevent the virus from spreading.

More research will be needed to determine what role, if any, the presence of the virus in saliva plays in the spread of the Zika virus, the scientists said.

"The important thing is now to determine whether the virus in saliva and urine can transmit the virus to others. We'll just have to wait and see," said William Schaffner, an infectious disease researcher at Vanderbilt University.

While other viruses can be found in saliva, such as the human immunodeficiency virus (HIV), it is not spread that way, Fauci noted.

Mosquitoes clearly remain the main way the Zika virus is transmitted, says Thomas Frieden, who heads the Centers for Disease Control and Prevention.

"I think it's important to step back and emphasize that Zika is a mosquito-borne virus and the overwhelming majority of cases are spread by mosquitoes," Frieden told reporters during a briefing Friday morning.

The CDC did, however, issue new guidelines for how pregnant women should protect themselves from getting infected, following a report out of Dallas this week that Zika had been spread through sexual contact in one case.

The CDC recommends that pregnant women whose sexual partners live in or have traveled to places where the virus is spreading should either abstain from sexual activity or "consistently and correctly" use condoms during vaginal, oral or anal sex.

Women who are not pregnant should consider doing the same if they are "concerned about sexual transmission," according to the guidelines.

The Zika virus is spreading rapidly through many parts of Latin America and the Caribbean. Concern has spiked because the virus is apparently causing some babies in Brazil to be born with microcephaly, a condition marked by small heads and brains. It has also been linked to Guillain-Barre syndrome, which can cause paralysis.

Mosquitoes that can transmit the Zika virus are found in the U.S., but so far aside from the Dallas sexual transmission case all the infections reported in the continental United States have been among travelers who got infected elsewhere. The CDC said there have been 50 such cases so far.

While officials say mosquitoes could spread the virus in the U.S., they are optimistic there will not be widespread transmission because of several factors, most notably good mosquito-control efforts.

But Frieden stressed that there is a lot that remains unknown about Zika.

"The situation is evolving rapidly, and as we learn more we'll share more so Americans can best decide how to protect their health," Frieden says.

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