If you're pregnant, avoid travel to a place where Zika is spreading.
The microcephaly link remains unconfirmed definitively, Frieden said, "but evidence of a connection is growing stronger nearly day-to-day".
A pregnant woman in Los Angeles County who traveled overseas has tested positive for the Zika virus, county officials said in a news release Friday.
Across the US thousands of college students, members of faith organizations, healthcare professionals and others are now planning spring trips to warmer locations for fun or charity work.
Montana's first case of a Zika virus infection has been diagnosed in Missoula County.
The CDC is following 10 other pregnant women. Pregnant women who must go should speak with their doctor first and do everything they can to avoid being bitten by mosquitoes during their time in Brazil. The advisory is complicated, as women often don't know that they are pregnant until several weeks into gestation and because health officials aren't sure how long the Zika virus stays in semen.
Scientists are working to confirm that the virus causes microcephaly, which has not been reported in other nations that have recently reported elevated Zika activity.
Because the couples were not living in locations where Zika is known to be spread by mosquitos, it's presumed the virus was spread through unprotected sex.
The CDC has identified many countries where there are known, active Zika virus transmission, but there's two main areas that you want to pay attention to. "The most serious risk of this virus is to pregnant women", he said. The one pregnant woman who reported Zika symptoms during her third trimester of pregnancy delivered a healthy infant, the report said.
Brazil has confirmed more than 580 cases of microcephaly, and considers a lot of them to be related to Zika infections in the mothers.
Babies with microcephaly, or abnormally small heads, can have a range of problems, including seizures, developmental delay, feeding problems and hearing loss.
"We did not expect to see these brain abnormalities in this small case series", Dr. Denise J. Jamieson, a leader in the pregnancy and birth defects group of the CDC's Zika Virus Response Team, told reporters in a phone briefing.
Zika infection is also believed to increase the risk of developing Guillain-Barré syndrome, a progressive paralysis from which most people eventually recover.