The devil in the dengue details Along with their mosquito vectors, dengue viruses are spreading worldwide to infect millions of people. For a few, subsequent infection results in lethal hemorrhagic disease. Katzelnick et al. used antibody-binding data to map structural divergence and antigenic variation among dengue viruses. Comparing results in monkeys and humans, the viruses approximately clustered into the four known groups. However, the four virus groups showed as much antigenic distance within a group as between groups. This finding helps explain why immune responses to dengue are highly variable, and it has complex implications for epidemiology, disease, and vaccine deployment. Science , this issue p. 1338