CDC held a media telebriefing Friday evening related to "interim travel guidance" for areas with ongoing Zika virus transmission. Lyle R. Petersen, the public health agency’s director of vector-borne diseases, said the virus is “spreading fairly rapidly through the Americas.”

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The CDC said the special precautions are aimed at pregnant women (in any trimester). Women who are trying to become pregnant should consult with their doctors.

Global health officials are closely monitoring the spread of the virus, which is suspected of causing brain damage in thousands of babies born in Brazil. Canadian infectious disease specialists have already warned that the Summer Olympics, which will draw thousands to Rio this August, could be a catalyst for spreading the virus globally.

There is no vaccine to prevent or medicine to treat Zika. Individuals can only protect themselves by taking steps to avoid mosquito bites. The most common symptoms of infection are fever, rash, joint pain and red eyes. Most people experience mild illness, with symptoms lasting from several days to a week.

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Researchers in Brazil found evidence of the virus in amniotic fluid taken from two pregnant women who were carrying fetuses diagnosed in utero as having microcephaly, a rare condition of an abnormally small head. It is associated with incomplete brain development.

Brazil's health ministry said Tuesday that 3,530 babies have been born with that condition since October. The number there was less than 150 in 2014. Zika virus was confirmed in Brazil last May.

Earlier this week, CDC scientists found evidence of the virus in the brain tissue of two Brazilian infants with microcephaly who died 24 hours after birth and in the placentas of two Brazilian women who miscarried fetuses with the condition. Earlier in their pregnancies, all four women had exhibited the fever and rash symptoms associated with the illness, Petersen said.

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In an interview Thursday, Petersen noted that the latest test results show "increasingly strong evidence of a link" between the virus and this congenital condition.

Of special concern, he said, is the high infection rate in previous outbreaks of the disease. After a Zika outbreak on the island of Yap in Micronesia, officials found that over 75 percent of the population became infected.

"So one of our concerns is this virus spreading to large numbers of people," he said. "And the potential association between Zika and microcephaly is of extreme concern."

Major outbreaks have been relatively rare, although French Polynesia experienced a large outbreak in 2013-2014 that caused an estimated 30,000 infections. Authorities didn't notice cases of microcephaly at the time, but when they later reviewed the records, they found 12 instances of birth defects resembling it. The disease's typical occurrence in a population would be zero to one cases, Petersen said. French authorities are investigating.

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In December, Puerto Rico reported its first confirmed Zika case. A Houston-area woman who traveled in November to El Salvador has been diagnosed with the virus, Texas health officials recently said. There have been no reports of cases transmitted locally in the United States.

The virus is transmitted to people primarily through the bite of an infected Aedes species mosquito -- the same kind that spreads the dengue and chikungunya viruses. It bites aggressively during the day and prefers to bite people. By comparison, the mosquito that carries West Nile virus prefers to bite in the evening and seeks birds more than humans.

Tips for travelers to prevent getting bitten by mosquitoes, according to CDC:

Wear long-sleeved shirts and long pants.

Use insect repellent approved by the Environmental Protection Agency as directed.

DEET (products include Off!, Cutter, Sawyer and Ultrathon) Picaridin, also known as KBR 3023, Bayrepel, and icaridin (products include Cutter Advanced and Skin So Soft Bug Guard Plus, as well as Autan outside of the United States) Oil of lemon eucalyptus (OLE) or PMD (products include Repel) IR3535 (products include Skin So Soft Bug Guard Plus Expedition and SkinSmart). Use products with a higher percentage of the following ingredients:

