Update (1/22/16): Today, the CDC listed eight additional locations with Zika virus travel alerts. The locations are: Barbados, Bolivia, Ecuador, Guadeloupe, Saint Martin, Guyana, Cape Verde, and Samoa. As with the agency's earlier guidance, the new advisories urge pregnant women and women planning to become pregnant to consider postponing travel to these areas. The original story appears below.

With mounting evidence that the mosquito-spread Zika virus is behind the skyrocketing numbers of severe birth defects in Brazil, the Centers for Disease Control and Prevention late Friday cautioned pregnant women and women planning to become pregnant to postpone travel to a set of Latin American and Caribbean countries and territories experiencing Zika outbreaks.

“Until more is known, and out of an abundance of caution, CDC recommends special precautions,” the agency said. The advisory relates to 14 countries and territories where Zika has newly spread: Brazil, Colombia, El Salvador, French Guiana, Guatemala, Haiti, Honduras, Martinique, Mexico, Panama, Paraguay, Suriname, Venezuela, and the Commonwealth of Puerto Rico.

The warning appears to be the first time the agency has ever recommended pregnant women avoid specific areas due to outbreaks. But health experts at the agency felt the “enhanced precaution” was prudent after new evidence directly linked the virus to four cases of microcephaly, in which babies are born with abnormally small heads and brains. The condition can be fatal.

As the agency made the announcement, the Hawaii State Department of Health confirmed the first US case of a baby born with microcephaly and evidence of a Zika infection. The newborn’s mother likely became infected with the virus while she was living in Brazil earlier in 2015 and the baby acquired the infection while in the womb, the department reported.

While there have been several cases of Zika in the US, all of the people stricken were infected outside of the country and returned with the virus. So far, there is no evidence to suggest that the virus is circulating in US mosquito populations or that it can transmit from person-to-person.

Zika, originally identified in Uganda in 1947, is mostly known for causing a mild dengue-like disease—with symptoms including rashes, fever, muscle pain, and weakness—and has never before been linked to such a birth defect. But, after Zika’s mosquito hosts began setting up shop in new locales, the virus arrived in Brazil last year. And it was swiftly suspected of being involved with an explosion of microcephaly cases.

Escalating quickly

Brazil reported its first case of Zika in May and since then Brazilian health authorities estimate that 500,000 to 1.5 million people have fallen ill with the virus. Amid that rampant viral outbreak, the country saw a more than 20-fold increase in microcephaly cases. The birth defect is generally rare and linked to genetic factors, in utero exposure to toxic chemicals, or certain infections, including rubella and chicken pox.

In 2014, there were 147 cases of microcephaly in Brazil. But from October 2015 to January 2016, there were more than 3,500 suspected cases, according to Brazilian health authorities.

The authorities found traces of Zika virus in pregnant women who gave birth to babies with the defect, as well as in amniotic fluid. Recently, the CDC found more direct evidence to support a link, CDC spokesperson Candice Hoffmann told Ars. The agency found that two fetuses who died in the womb were infected with the virus. Additionally, two babies born with microcephaly who were carried to full term but died after birth had traces of Zika virus in their brain tissue. The mothers in all four cases reported that during their pregnancies they got sick with a rash-causing illness consistent with Zika.

The new data was enough to convince the CDC to caution pregnant women headed to Zika-stricken areas. But, it’s still does not definitively prove that Zika causes or contributes to the defects. To do that, researchers must conduct “intensive investigative research, including careful case-control and other epidemiologic studies as well as attempts to duplicate this phenomenon in animal models,” Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, and David Morens, a medical historian at the institute, report.

Meanwhile, health experts are scrambling to get out ahead of Zika’s spread.

It’s been known to circulate in mosquito populations in several African and Asian countries for years. But amid warming global temperatures and increased global travel and trade, its mosquito hosts of the Aedes genus have spread to new areas, particularly countries in the Western Hemisphere. Authorities in Brazil are now instituting new campaigns to squash mosquito populations there, which are also spreading dengue and chikungunya infections.

In the US, health experts have nervously discussed the possibility that Zika could become established in mosquito populations here and start spreading. "Many areas in the United States have mosquitoes that can become infected with and transmit Zika virus,” the CDC’s Hoffman told Ars. “However, recent chikungunya and dengue outbreaks in the United States suggest that Zika outbreaks in the U.S. mainland may be relatively small and focal.”

So far, there is no specific treatment or vaccine for Zika.

In light of this new threat, Fauci and Morens said “we clearly need to up our game with broad and integrated research.”