Since Latin America’s Zika outbreak began, in early 2014, the virus has radiated northward from Brazil, lapping up against the shores of this country, exiting our airports in the bloodstream of passengers returning from vacation and work abroad. So far, there have been four hundred and seventy-two confirmed cases of travel-associated infection in the continental United States. The mosquitoes that carry the virus are already here, endemic in much of the South, and, though they have been quiet through the winter, warmer weather is stirring them out of dormancy. Now the question is how extensively Zika will spread. Will it infect pregnant mothers here, and will their infants suffer the same apparent effects as those in Brazil—the birth defect known as microcephaly , which severely hinders brain development? “Everything we look at with this virus seems to be a bit scarier than we initially thought,” Anne Schuchat, the principal deputy director of the U.S. Centers for Disease Control and Prevention, said recently. A new study, published in this week’s issue of the journal Nature, may help provide an answer.