The Zika virus has gone from an obscure disease to an international public health emergency.
Researchers have been able to trace the gradual spread of Zika — slowly for decades and then, in the words of World Health Organization head Dr. Margaret Chan, "explosively" since 2015, when it was first detected in Brazil. Now the virus has reached more than 20 countries and territories in the Americas. In Brazil, there have been 500,000-plus cases. Thousands of babies have been born in Zika-affected regions with the birth defect microcephaly (although a causal relationship has not yet been scientifically established).
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