This story is part of a partnership between MedPage Today and VICE News.
A virus suspected of causing infants to be born with abnormally small heads and underdeveloped brains has arrived in a United States territory, and it may be headed north.
After the Zika virus was detected in Puerto Rico last week, the Centers for Disease Control and Prevention issued a travel advisory. The low-level warning instructs travelers to avoidmosquito bites by wearing long sleeves and pants, using insect repellent, and sleeping in air-conditioned rooms or under bed nets. There is no vaccine for the virus.
"Zika is new on everyone's radar screen," said Dr. William Schaffner, chair of preventive medicine at Vanderbilt University. "Given that there are questions about it, I think the CDC has done the right thing."
Zika is a virus spread by mosquito bites that is known to cause fever, rash and joint pain, according to the CDC. But there are new fears about a much more serious side effect.
In late November, the Brazilian Ministry of Health announced a possible link between a tenfold increase in the number of babies born with microcephaly and signs of Zika virus infection. Babies with microcephaly have small heads because their brains did not completely develop in the womb.
"For us, it's very new. This has never happened before. It's a war operation," Luciana Albuquerque, executive secretary for health surveillance in the Brazilian state of Pernambuco, told VICE News in November. "It's as much about receiving and supporting as knowing the cause."
Related: Are Mosquitoes Behind an Increase in Brazilian Babies Born with Abnormally Small Heads?
There were 1,761 babies in Brazil born with microcephaly as of December 5, according to the World Health Organization, and 19 of them had died. The health crisis was such that in late December officials took the unusual step of discouraging Brazilians in hard-hit areas to become pregnant.
"It's a very personal decision, but at this moment of uncertainty, if families can put off their pregnancy plans, that's what we're recommending," said Angela Rocha, the pediatric infectologist at Oswaldo Cruz Hospital in the state of Pernambuco, to CNN. The state had already seen more than 900 microcephaly cases.
According to the CDC, some of the Brazilian babies with microcephaly tested positive for Zika, but others tested negative. No studies have yet established a link between Zika and the tenfold increase in microcephaly, which has many causes including genetics, an infection in the mother during pregnancy, and exposure to toxic chemicals during pregnancy.
Dr. Frank Esper, a pediatric infectious diseases expert at UH Case Medical Center in Cleveland, Ohio, said it's important to remember that it will be years before scientists can determine whether Zika virus causes mirocephaly. But he suspects health officials are already looking at the brains of babies with microcephaly to see what they have in common, and we'll know more in the coming weeks and months.
"That's going to be next piece of the puzzle," he said. "A few head scans so we can figure out what's going on."
Although babies with microcephaly have a greater risk of brain development problems, some go on to live normal lives, Esper said.
"Babies brains are so able to compensate even when they're substantially damaged, they're able to reroute neurons," he said. "Completely other parts of the brain are able to take over."
Still, the CDC's advisory said that pregnant women traveling to areas where Zika virus has been discovered should avoid mosquitos as a precaution.
"I've been called by people going to Brazil and pregnant and wondering whether they ought to go," Schaffner said. "It's so dramatic and so poignant and so tragic that people are really, really worried about that."
Zika was first discovered in 1947 when a monkey in Uganda became infected with the virus, but it wasn't reported in humans for another 20 years. The virus remained in Africa and Asia as part of small outbreaks until 2007.
Starting in October 2015, Zika cases have surged with outbreaks in Brazil, Colombia, Suriname, El Salvador, Guatemala, Paraguay, Mexico, Venezuela, Panama, Cape Verde and Honduras, according to WHO. It is among other mosquito-borne diseases, including dengue fever and chikungunya, that were on the rise in 2015, said John Brownstein, director Health Map, a web tool that collects infectious disease information from thousands of sources.
Related: The Outbreaks of 2015: Ebola, MERS, and Measles at the Happiest Place on Earth
The fact that a Zika case has been reported on Puerto Rico signals that the virus is in the local mosquito population.
"Either an infected mosquito can be transmitted in an aircraft or ships or infected people can come over, and then are bitten by our own mosquitos here and you set up the cycle," Schaffner said. "The latter is a bit more likely in this instance."
The breed of mosquito that carries Zika is called Aedes aegypti, and it is found about as far north as Georgia in the United States, Schaffner said. What remains to be seen is whether the virus can live in a related mosquito breed that's even more prevalent in North America, Aedes albopictus.
"We may be hearing about this more as we get through our winter and then get into spring and things warm up and mosquitos start to breed again," Schaffner said.