Zika's rampage last year in Brazil caused an explosion of infections and inflicted a crippling neurological defect on thousands of babies — an effect never seen in a mosquito-borne virus.
It also presented a mystery: why had a virus that had been little more than a footnote in the annals of infectious diseases taken such a devastating turn in the Americas? How had Africa and Asia, where Zika had quietly circulated for decades, escaped with no reports of major outbreaks or serious complications?
Scientists initially theorized that Zika's long tenure in Africa and Asia may have conferred widespread immunity. Or, perhaps older strains were less virulent than the one linked in Brazil to more than 2,100 cases of microcephaly, a birth defect characterized by arrested brain development.