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The Zika virus could spread to four million people this year. So what is it?

It was discovered in Uganda in the 1940s, then detected in a few other places over the years.

It broke out in the Americas and the Caribbean in 2015.

Zika is transmitted through a particular species of mosquito that lives mostly in tropical regions.

That includes parts of the U.S. There was evidence that the virus could be transmitted through other means too.

Zika has been linked to paralysis and birth defects.

Brazil has seen a spike in babies born with a certain kind of brain damage.

Officials are now warning pregnant women not to travel to places where there's an active Zika outbreak.

Generally though, Zika symptoms are mild, and most people infected don't notice anything.

There is no treatment for the virus, and officials say finding a vaccine could take up to a decade.