Zika is more pernicious than public health officials anticipated. At present, it is circulating in more than 50 countries. And as of mid-May, seven countries or territories have reported cases of microcephaly or other serious birth defects linked to the virus, which is trans­mitted by mosquito bite, blood transfusion or sexual contact with an infected human. It can also be passed from mother to fetus during pregnancy.

Despite Zika’s vast range over almost 70 years, there is little genetic difference among the various strains, according to an analysis by re­­searchers at the University of Texas Medical Branch at Galveston. For example, the strain currently in the Americas and another previously detected in French Polynesia are practically indistinguishable from each other (group in white box). If the virus has changed so little over time, why is it rearing its ugly head now? Scientists are not sure yet, but new experimental work in mosquitoes suggests that the virus was capable of causing detrimental health effects and outbreaks all along. Therefore, it is unlikely mutations enabled new abilities. Instead public health officials probably did not understand Zika’s potential because the virus circulated mostly in remote locations until recently.