On Friday, the US Centers for Disease Control and Prevention issued a travel alert, warning pregnant women to avoid visiting countries where the Zika virus has been circulating.
Zika, which is carried from person to person by mosquitoes, has been linked to birth defects and deaths in newborns in Brazil.
Though that country has been battling the largest Zika outbreak, with more than a million people infected, the virus has also been detected in Colombia, El Salvador, French Guiana, Guatemala, Haiti, Honduras, Martinique, Mexico, Panama, Paraguay, Suriname, Venezuela, and the Commonwealth of Puerto Rico.
Researchers think the virus may cause microcephaly, a congenital condition that's associated with a small head and incomplete brain development in newborns.
Though the CDC warns that "additional studies are needed to further characterize this relationship," it's advising pregnant women, and women who are trying to become pregnant, to "consider postponing travel to the areas where Zika virus transmission is ongoing."
Those who must travel to countries where the virus is spreading should consult their doctors, the CDC advised. Other travelers should take precautions to avoid mosquito bites by wearing long sleeves and using bug repellent.
Why the Zika virus is so freaky: It may lead to babies with abnormally small heads
Until very recently, if you asked a health researcher about the Zika virus you would probably get one of two answers.
First: They've never heard about it. Though Zika was discovered in 1947 (in the Zika forest in Uganda), it hasn't bothered humans much in all these years. Until 2007, there had been fewer than 20 documented cases, followed by mostly local outbreaks in the South Pacific and French Polynesia.
Second: Until recently, most researchers didn't think it was that dangerous. In most people, Zika only seemed to produce mild symptoms — rash, headaches, pain in the bones, and fever — that usually show up between three and 12 days after a mosquito bite. These symptoms go away within a week, and one in four people don’t even develop any illness after being infected with the virus.
But over the past year, public health officials have started to wonder whether the virus is more harmful than they previously thought.
Consider Brazil. Health officials there estimate that some 1.5 million people have been infected with Zika in the last year alone, making it ground zero for the largest outbreak in the Americas. (Researchers speculate that the disease may have arrived with World Cup travelers in 2014.)
And here's where things get worrisome. Though Brazil usually sees several hundred cases of microcephaly each year, in 2015 officials documented at least 3,500 cases — that's nearly 20 times more than the usual number.
The uptick in microcephaly started appearing a few months after the virus was identified, which is why Brazilian health officials believe there may be a terrible link between Zika and the birth defect. When they studied the amniotic fluid in pregnant women carrying babies with microcephaly, they found Zika.
More recently, scientists at the CDC provided the strongest evidence yet of a potential link between Zika and microcephaly: They found Zika virus in the brains of two babies with the condition who died within 24 hours of being born.
There’s also a chance that Zika causes Guillain-Barré syndrome, a very rare neurological syndrome that attacks the muscles and can cause paralysis. Zika has also been linked to more than 40 deaths. With such a large outbreak and so many cases to study, researchers think they might be seeing that the virus is actually more dangerous than they previously imagined.
That said, there's a lot the scientific community still has to learn about the virus. Because it was previously so rare, it wasn't very well studied. And it's possible that something else is causing the rise in microcephaly in Brazil — even another virus.
To learn more, read our Zika virus explainer.