On Feb. 1, the World Health Organization declared the spread of Zika virus across Latin America a “public health emergency of global concern.” Zika is a mosquito-borne virus related to the dengue and yellow fever viruses. In most cases, infection by Zika has no symptoms, which means the disease is usually not diagnosed. Only 20 percent of Zika cases are accompanied by symptoms such as fever and rash. The disease, which originated in Brazil, has spread to almost 30 other countries and territories, including the United States, where at least 82 cases have been reported.