Fears of the Zika virus spreading in the United States have reached critical mass as local public health officials and environmental advocates work to contain cases reported in Florida this week. Authorities have been spraying insecticides in Miami’s Wynwood arts district, where more than a dozen people have become infected with the first mosquito-transmitted cases in the mainland U.S. The country has been girding for months against the epidemic coursing through Latin America and the Caribbean. Zika poses a serious risk to pregnant women and their babies such as severe brain-related defects, including disastrously small heads.