There's a new virus spreading in South America, and it's been documented in the United States, but only among travelers.
It's called the Zika virus, and there is no cure.
On Monday afternoon during a call with reporters, the virus was declared a "public health emergency of international concern" by the World Health Organization, establishing Zika as a serious threat.
Here's a rundown of the good and bad news about the virus.
First, the good news: Zika is rarely fatal. Symptoms are similar to those of a cold or fever.
The bad news: It may be linked with a dangerous birth defect known as microcephaly, where babies are born with abnormally small heads.
Even worse news: There is no rapid diagnostic test to detect the virus in a newly-infected person.
So, you have a quickly spreading virus, that can't be detected that can lead to birth defects in newborns.
Last week, World Health Organization officials said the virus was “spreading explosively” in the region and President Obama voiced his concerns for the Zika virus here in the US, calling for more research into ways to stop the spread of the disease.
This is not the first time the alarm bells have been sounded. Earlier in January, Peter Hotez, Dean of the National School of Tropical Medicine at Baylor College of Medicine, told NBC News that he was "very worried about Zika."
Hotez, who's also the Director of the Texas Children's Hospital Center for Vaccine Development, said that while a single tourist is unlikely to be the cause of an outbreak here, some American cities could be vulnerable to Zika's spread.
"We have to act now," Hotez said.
The problem with Zika: Low-level symptoms and potentially serious consequences
Hotez added that one of the biggest issues with the Zika virus, which is spread by a certain species of mosquito called Aedes aegypti, is that it "tends to produce low-level symptoms." They include fever, rash, join pain, and red eyes. But there's a bigger problem, too: Once infected, only about 20% of people with Zika ever show those symptoms, according to the CDC. Plus, the illness is typically mild — symptoms usually last anywhere from several days to a week, and hospitalization is rarely necessary.
But the virus, while not necessarily damning in and of itself, has been linked with a far more concerning problem: babies born with abnormally small heads, a serious condition known as microcephaly. After some mothers showed symptoms of the virus during their pregnancy, their babies were born with the condition.
Since the outbreak of the Zika virus in April 2015, Brazil has documented 4,180 cases of the condition in babies born to women who were infected during their pregnancy — 20 times the rate of the previous year and a 7% increase from the number recorded just last week.
Where the virus is now
So far, the disease has been identified in a number of states in people who recently traveled to areas where the virus is being transmitted locally. Local transmission of the virus, which is spread by mosquitoes, has been documented in Puerto Rico and the US Virgin Islands as well.
Here are the 24 countries and territories where the virus had been transmitted locally as of Wednesday:
<