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U.S. health officials are considering a travel warning about Zika virus, the once obscure virus rapidly spreading across Latin America and the Caribbean that experts fear may cause birth defects.

It’s not clear what the officials will tell people just yet, because so little is known about the virus, which is spread by mosquitoes. But evidence is growing that it may cause a catastrophic birth defect called microcephaly.

What is clear is that it’s likely to spread much more widely. It’s also clear that it will take weeks, if not months, to get all the answers. In the meantime, there’s no vaccine against Zika, a viral cousin of dengue fever, and no treatment for it.

“We are in the process of developing a travel warning not only for pregnant women, but for everybody,” said Dr. Lyle Petersen, who heads the Center for Disease Control and Prevention’s division on diseases carried by mosquitoes and other insects and animals.

“We are in the process of developing a travel warning not only for pregnant women but for everybody."

Microcephaly — an abnormally small head and brain — can kill babies, cause miscarriages or cause severe and untreatable handicaps.

Until recently, Zika wasn’t on the radar screens because it only caused very mild illness.

“Zika does cause a dengue-like illness,” Petersen said. “It causes fever, headache, skin rash, red eyes, muscle ache, that sort of thing. Those illnesses are pretty mild.” And they look like the symptoms of many other viral infections, including dengue, which is far more dangerous and deadly.

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It had also been suspected of causing Guillain-Barre syndrome, a rare and sometimes dangerous reaction that often occurs after viral infections.

A researcher looks at Aedes aegypti mosquitoes kept in a container at a lab of the Institute of Biomedical Sciences of the Sao Paulo University, on Jan. 8, 2016 in Sao Paulo, Brazil. NELSON ALMEIDA / AFP - Getty Images

It had also been tied to Rubella, also known as German measles, which can cause microcephaly, learning disabilities, heart disease and other defects. And the most dangerous time is during the first trimester, when women might not yet realize they are pregnant. It’s one reason why doctors stress rubella vaccination and why pregnant women are tested to make sure they’re immune.

Other viruses related to Zika, dengue for instance, are not known to cross the placenta and affect a fetus. Petersen said there is now some evidence that Zika is doing this.

CDC tested tissue taken from two babies who died of microcephaly in Brazil.

“We found virus in the heads of those children,” Petersen said. “That’s pretty good evidence that, at least in those children with microcephaly, there was Zika virus in their brains.”

Other researchers found evidence of the virus in the placentas of two babies that were miscarried. And Brazilian researchers have found evidence of the virus in amniotic fluid — which supports and surrounds a fetus.

Their reports suggest severe brain damage. Some of the babies had brain calcifications, which suggests something killed the brain tissue.

Brazil is experiencing an explosion of microcephaly, from a few hundred cases reported a year to close to 4,000 over the past year.

But it’s not easy to test for Zika. A quick blood test can get Zika mixed up with dengue, which is a longtime resident of the region and all tropical areas. A test called PCR is needed to detect Zika, and it takes longer and is more expensive that a quick test.

So it’s hard to know just how many people are infected or have been infected. "We need to investigate more," Petersen said.

When another virus called Chikungunya started spreading across the Caribbean and Central America in 2013, the CDC issued a travel warning that simply cautioned people traveling to affected areas to take precautions against mosquito bites: staying inside, wearing long sleeves and using insecticide. So far, that’s what the Pan American Health Organization (PAHO), an arm of the World Health Organization, has done.

"Zika is still a pandemic in progress."

CDC are now deciding what they should tell people about Zika, how wide an area to include in any warning and whether it should go beyond the standard cautions about mosquito-borne diseases.

Zika's been reported across the warm parts of Latin America, from Brazil to Mexico and even in Puerto Rico. Travelers have occasionally carried it to the U.S. and some experts believe it's only a matter of time before it settles into warm regions such as south Texas, Florida and Louisiana.

Hawaii just battled an outbreak of dengue carried by travelers. It's becoming clear that viruses carried by mosquitoes are a growing threat, said Dr. Anthony Fauci, head of the U.S. National Institute for Allergy and Infectious Diseases. "To respond, we urgently need research on these viruses," he wrote in Thursday's issue of the New England Journal of Medicine. And researchers need to develop broadly effective antiviral drugs that can treat more than one virus.

"Zika is still a pandemic in progress," Fauci said. "We clearly need to up our game with broad and integrated research."

PHOTOS: Brazil Blames Virus for Causing Birth Defects