A single genetic change that occurred in 2013 may explain how Zika acquired the ability to attack fetal nerve cells, causing a severe birth defect in babies whose mothers were infected while pregnant, Chinese and U.S. researchers reported on Thursday.
Scientists have posited many theories about why Zika, a mosquito-borne virus that had been linked with only mild symptoms since its discovery in 1947, could suddenly be associated with thousands of cases of the birth defect known as microcephaly, as it was in Brazil in 2015.
That outbreak prompted the World Health Organization to declare Zika a public . . .
