By David Quammen It is "spreading explosively" in the Americas and may be the next public health emergency. Scary new viruses emerge abruptly in our modern world, provoking stark headlines and demands for bold government action—but in most cases the causes are complex and have developed, unnoticed, over years or decades. That's true again for Zika, a virus unknown to most people until recent days, and now suddenly the subject of somber warnings from the Centers for Disease Control and Prevention and the World Health Organization, which announced on Thursday that the virus is "spreading explosively." The alarm stems from an epidemic of birth defects in Brazil, which may be linked with Zika virus infection of mothers during pregnancy. Amid this furor, it's worth distinguishing fact from supposition and placing the Zika phenomenon in a broader context. This virus was first isolated in Uganda in 1947, within a small enclave called Zika Forest, near the west shore of Lake Victoria, where