From October 2015 to January 2016, there were almost 4,000 cases of babies born with microcephaly in Brazil. Before then, there were just 150 cases per year. The suspected culprit is a mosquito-borne virus called Zika. Officials in Colombia, Ecuador, El Salvador and Jamaica have suggested that women delay becoming pregnant. And the Centers for Disease Control and Prevention has advised pregnant women to postpone travel to countries where Zika is active.