IFNAR1 and infection: In order to assess the effect of type I IFN responses on the pathogenesis of O. tsutsugamushi infection, we first evaluated the survival rate of wild type and mutant mice deficient in a receptor subunit for IFN-α and IFN-β (IFNAR KO) after intraperitoneal infection with fatal dose (5 × LD50) of O. tsutsugamushi (Figure 3A).