Double whammy Dengue and Zika virus epidemics have been lapping each other around the globe. These are closely related mosquito-borne viruses with about 40% homology within the envelope protein. We know that subsequent dengue infections bring a risk of antibody-dependent disease enhancement. Whereas emphasis has been placed on how prior dengue immunity affects Zika infection, little is known about how prior Zika immunity may affect dengue disease. Katzelnick et al. have been following a well-characterized and established pediatric cohort in Nicaragua who were serially exposed to both flaviviruses in recent years (see the Perspective by Clapham). This study shows not only that a previous history of just one round of dengue is a problem but also that prior Zika immunity creates an increased risk for severe dengue virus sereotype 2 infection. By contrast, multiple infections raise antibodies to protective levels. Science , this issue p. 1123 ; see also p. 1055