When the Zika virus became a global terror two years ago — inflicting severe birth defects on the babies of pregnant women who were infected with the virus and alarming health officials worldwide — scientists were mystified. How did such an obscure, relatively harmless pathogen that had been known for more than half a century suddenly blossom into a monster virus? Was it something about Zika’s new hunting grounds when it spread from Africa to South America and the Caribbean?

A team of researchers from the U.S. and Nicaragua has found evidence that suggests people infected a second time by the dengue virus may experience a more severe form of dengue fever if their antibody levels are at a certain level. In their paper published in the journal Science, the team describes their study of the disease in children in Nicaragua over the course of 12 years and what they found. Dengue fever is a malady caused by infection of the dengue virus. On first infe