Pregnant women should consider avoiding travel to 13 South and Central American countries and Puerto Rico for the time being because of the risk of contracting Zika virus, the Centers for Disease Control and Prevention said Friday evening.

The little-known virus is believed to be responsible for a large surge in birth defects seen in recent months in Brazil, where the virus first started spreading last May. Health officials are concerned that the virus is also linked to a surge in cases of microcephaly, a condition in which babies are born with smaller than normal heads and underdeveloped brains.

In an unusual Friday night news conference, experts from the CDC also said the agency is advising women hoping to become pregnant to talk to their doctor if they are planning to travel to any of the identified countries and to take stringent precautions to avoid being bitten by mosquitoes if they go.

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Pregnant women who cannot postpone travel to the countries listed on the travel alert should also speak to their doctors and take steps to protect themselves against mosquito bites, the experts said.

The countries listed are Brazil, Colombia, El Salvador, French Guiana, Guatemala, Haiti, Honduras, Martinique, Mexico, Panama, Paraguay, Suriname, Venezuela as well as the Commonwealth of Puerto Rico. Puerto Rico reported its first locally acquired case of Zika infection on Dec. 31.

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CDC
Countries that have past or current evidence of Zika virus transmission. CDC

Dr. Lyle Petersen, director of the CDC’s division of vector-borne diseases, said it’s too early at this point to say how long the travel advisory will remain in effect.

Zika virus, which can be transmitted by mosquitoes, has exploded onto the world stage in recent weeks. Alex Hogan/STAT

But that question will be asked with increasing frequency as the 2016 Summer Olympics in Rio de Janeiro approaches. The Rio games are slated to run from Aug. 5 to Aug. 21.

Zika virus transmits in a cycle involving mosquitoes and people. A mosquito becomes infected by taking blood meals from people with virus in their blood. Once infected, the mosquito passes the virus along.

The virus spreads geographically when a person who is infected travels to another setting and is bitten there, potentially starting a transmission cycle in the new locale. It could also happen if an infected mosquito was transported to a new location, in shipped goods, for instance.

Zika virus is transmitted by Aedes mosquitoes — Aedes aegypti and possibly Aedes albopictus. The former are found in the southern United States while the latter are hardier and have been found as far north as New York. A study released Thursday said Florida and a portion of southeastern Texas are particularly vulnerable to local transmission of Zika virus.

Though first identified in 1947, the virus has not been the subject of much research; until recently it wasn’t thought to be a source of serious human disease. Zika infection is generally mild, featuring flu-like symptoms and a rash.

But its reputation as essentially harmless has started to change. A huge Zika outbreak in Brazil last spring was unexpectedly followed a few months later by a startling increase in babies born with microcephaly.

A link between Zika infection in pregnancy and microcephaly in infants hasn’t been definitely established. In fact, coming up with conclusive proof will be difficult.

But there is a mounting evidence that experts find persuasive. Brazilian scientists found traces of the virus in amniotic fluid drawn from the wombs of two women who were discovered to be carrying fetuses with microcephaly.

And this week the Brazilian Health Minister reported that work done at its behest by the CDC provided additional evidence, which Petersen described in an interview earlier this week as “the strongest evidence to date of a possible link between Zika virus and microcephaly and other congenital abnormalities.”

Scientists at the CDC examined the brains of two infants who died within 24 hours of being born with microcephaly, as well as placenta samples from two women who miscarried fetuses with the condition.

Using two different types of tests, the CDC scientists found evidence of Zika virus in the brains of the newborns who died. Petersen said this provides “very strong evidence of infection in the brains of these two infants.” The placenta samples also tested positive for the virus by what’s known as a PCR test.

Petersen said more study is planned. Brazilian health officials will follow women who are pregnant to chart outcomes. And the CDC will partner with Brazilian public health scientists to do what is called a case control study, comparing the pregnancies of women who gave birth to a baby with microcephaly to similar women who gave birth to healthy babies.

The briefing to release the travel advice was scheduled for 3 p.m. Friday, but was postponed a few minutes before it was to start.

When experts were asked during the news conference if the CDC had been pressured not to issue this advice, a spokesperson jumped in to say that the situation was evolving and the agency needed more time.

She did not directly address the question of whether any parties — either within the US government or from countries named in the alert — had urged the CDC not to take this step.

But Dr. Martin Cetron, the CDC’s director of global migration and quarantine, acknowledged countries listed were warned about the pending announcement. Cetron said the agency does not like to “blindside partners.”

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