Project Summary Coxsackievirus B3 (CVB3) is a common infection that is spread by the fecal-oral route. Nearly all humans are infected with a coxsackievirus by the age of 5, and coxsackievirus is a major cause viral myocarditis, meningoencephalitis, and hand, foot, and mouth disease. Using a recently developed mouse model to study CVB3, I found that, similar to humans, male mice succumb to CVB3-induced disease, whereas female mice do not. Intriguingly, CVB3 replication in the intestine of male mice is enhanced and requires intestinal bacteria. Furthermore, CVB3 replication can be restored in female mice when they are given microbiota from male mice by fecal transplantation. This suggests, for the first time, that sex differences in intestinal bacteria alter viral replication. However, the mechanisms for this sex-bias remain unknown. Emerging data indicate that intestinal bacteria differ in males and females and that bacterial differences correlate with sex hormones, such as testosterone. These data suggest that bacteria and sex hormones may influence CVB3 replication in the intestine. The goal of this study is to identify intestinal bacteria that promote CVB3 replication and investigate the effect of sex hormones on intestinal CVB3 replication. These studies will shed significant insights into the intestinal environment required for coxsackievirus replication and will be beneficial to understanding human infections.