Zika virus (ZIKV) is a mosquito-transmitted flavivirus that is closely related to dengue virus (DENV). Since its discovery in 1947, ZIKV remained relatively unnoticed, causing small, local outbreaks primarily in parts of Africa and Asia, and was associated with minor symptoms, such as mild fever. However, in the last decade, ZIKV started to spread geographically across the Pacific islands, eventually reaching South America, where it caused an explosive outbreak that started in Brazil in 2015 and rapidly spread to other South and Central American countries. This has been accompanied by a startling link between ZIKV infection during pregnancy and the development of birth defects among fetuses and babies, including microcephaly. It is unclear what factors may have led to the massive ZIKV outbreak or the severe disease manifestations in the Americas, but one potential variable is that much of the at-risk population in the Americas has preexisting immunity to DENV. It is well documented that preexisting immunity to one serotype of DENV can alter the disease pathogenesis of a subsequent infection with a different DENV serotype, a phenomenon called antibody-dependent enhancement (ADE). Because ZIKV outbreaks have occurred in regions around the world where DENV is endemic and due to the high degree of relatedness of ZIKV and DENV, it is critical to understand and characterize the extent to which prior infection with DENV exacerbates ZIKV disease. In this application, we will focus on how DENV antibodies impact ZIKV infection in the context of pregnancy by utilizing three model systems (immunocompromised Stat2-/- mice, immunocompetent humanized STAT2 knock-in mice, and human placental explants infected with ZIKV ex vivo).