The World Health Organization has declared the Zika virus outbreak a "public health emergency of international concern."
Until last year, Zika, which is spread by mosquitoes, was mainly confined to Africa and Asia. But in 2015 the disease made the leap to the Western Hemisphere, affecting more than a million people in Brazil.
It has since spread across Latin America and the Caribbean, and experts expect that the US will also see active transmission of the virus by summer.
The virus causes fairly mild symptoms in adults, but doctors suspect it can be passed to unborn fetuses, and they're seeing a spike in birth defects correlated with the spread of Zika.
Watch the video above to learn more about the Zika virus and how it spreads.