The Center for Disease Control and Prevention has predicted that a quarter of Puerto Rico's population, some 3.5 million people, will be afflicted with the Zika virus within a year. They fear that 80 percent or more could be infected eventually. The C.D.C. has set up a permanent lab on the island and is attempting to conduct 100,000 blood tests a year. Officials have been prioritizing the protection of pregnant women, who, when afflicted, can have children with birth defects.