Coinfection with bacteria and viruses is an understudied area of microbiology, despite its potential to modulate pathogen abundance and host survival. We investigated the effect of bacteria on virus infection and developed an in vitro system to study the first step: viral internalization. Our studies show that multiple bacterial species promote the entry of a diverse panel of viruses into lung and gut epithelial cells. Bacteria expressing the toll-like receptor (TLR)5 agonist, flagellin, are most efficient at inducing viral uptake and studies using recombinant flagellin or aflagellate bacterial strains confirm that flagellin has pro-viral activity. Flagellin promotes epithelial cells to support virus entry via TLR5-dependent activation of NF-KB. To extend these observations and study the role of flagellin in the complete viral replicative lifecycle, we studied human immunodeficiency virus (HIV)-1 replication in T cells. Flagellin augments HIV-1 entry and promoter activity and increases the production of extracellular virus. The data presented in this thesis highlight a new role for bacterial flagellin to promote diverse virus infection of epithelial barriers and enhance the spread of HIV-1. This has significant implications for understanding how exposure to multiple pathogens can alter susceptibility to infection and its associated pathogenesis.