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The World Health Organization has declared Zika virus a global health emergency of international concern. It's projected that the virus could infect up to 4 million people this year.
The last global health crisis to rise to that level was the Ebola epidemic, which spanned 2014 and 2015. But unlike Ebola, which cycled in and out of the news for decades, few people had ever heard of Zika virus before last month. The mosquito-borne virus was known to cause mild symptoms like rashes and fevers, but in most infected people - four out of five - Zika didn't produce any symptoms at all.
That was before Brazilian health officials announced last month that they'd seen 4,000 cases over the last year of infants born with microcephaly, a severe birth defect causing small heads and under-developed brains, and that they believed the defects were linked to Zika.
The virus has been recently found in at least 24 countries, and there are reportedly at least 30 cases in the United States - all of them among people who traveled to high-risk regions. The spread of Zika has left many asking why now, and where it came from.
Scott Weaver, director of the Institute for Human Infections and Immunity at the University of Texas Medical Branch, joins Here & Now's Jeremy Hobson from Galveston, Texas, to answer these questions.
This segment aired on February 1, 2016.