A paper published by The Lancet Infectious Diseases today offers some answers. It shows the currently circulating strain of the Oropouche virus replicates far better in cell cultures than an older strain did—suggesting it may do better in humans as well. The data also suggest the current strain is different enough from earlier ones that people infected a decade ago have almost no immunity against today’s version. “This is not a new virus,” but it seems to “replicate faster, replicate better, and be more virulent.