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  　 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  　 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  　 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  　 mostly in remote locations until recently .