Bacteriophage are abundant at sites of bacterial infection， but their effects on mammalian hosts are unclear. We have identified pathogenic roles for filamentous Pf bacteriophage produced by () in suppression of immunity against bacterial infection. Pf promote wound infection in mice and are associated with chronic human wound infections. Murine and human leukocytes endocytose Pf， and internalization of this single-stranded DNA virus results in phage RNA production. This triggers Toll-like receptor 3 (TLR3)- and TIR domain-containing adapter-inducing interferon-β (TRIF)-dependent type I interferon production， inhibition of tumor necrosis factor (TNF)， and the suppression of phagocytosis. Conversely， immunization of mice against Pf prevents wound infection. Thus， Pf triggers maladaptive innate viral pattern-recognition responses， which impair bacterial clearance. Vaccination against phage virions represents a potential strategy to prevent bacterial infection.