Pregnant women who contract rubella during the first trimester have an 85 percent chance of delivering a child with severe, permanent defects of the eyes, ears, or heart. Before a rubella vaccine became available in 1969, one outbreak in Philadelphia in the early 1960s caused hundreds of spontaneous abortions, elective abortions, and children who were permanently disabled. Think of the fear engendered when Zika virus spread from South America into several southern states. People were horrified by television images of babies with abnormally small heads. But Zika virus, which causes birth defects in about 15 percent of first trimester infections, isn’t nearly as efficient at causing harm as rubella.