Zika virus has been declared a public health emergency. As many as 1.3 million persons have been affected in Brazil alone, and 20 countries or territories have reported local transmission of the virus during 2016. Because of the ease of air travel and international trade, further spread into regions where the virus is not endemic is likely, and transmission is probable in locations with competent mosquito vectors.However, uncertainty remains regarding aspects of the virus’s vectors, epidemiology, and pathogenesis. As the epidemic unfolds, evaluating incoming data critically will be necessary to separate fact from speculation.