Simple SummaryTo investigate whether the influenza virus infection affects the reproduction and transmission ofToxoplasma gondii, we attempted to identify the specific influencing factors. In this study, we found that decreased expression levels of IL-1β, IL-6, and IL-12 in the co-infected group were associated with the early immune responses of the host againstT. gondiitype II strain (Pru), which affected the division ofT. gondii(Pru). Moreover, the significant decrease in the CD4+/CD8+ratio indicated a weakened long-term immune killing ability of the host againstT. gondii(Pru) after IAV infection. In conclusion, our study showed that a mixed infection of IAV (influenza A virus) virus andT. gondii(Pru) co-infection was lethal in mice and that the two pathogens may act synergistically.AbstractThis study aimed to investigate the relationship between theT. gondiitype II strain (Pru) and respiratory viral infections, specifically focusing on the co-infection with PR8 (influenza A/Puerto Rico/8/34). In this study, we found that the number ofT. gondii(Pru) in the lungs of co-infected mice was significantly higher and lesions were more severe than those in the group infected withT. gondii(Pru) alone, whereas IAV (influenza A virus) copy numbers of co-infected and PR8 alone infected groups were negligible, suggesting that infection with IAV increased the pathogenicity ofT. gondii(Pru) in mice. The invasion and proliferation assays demonstrated no significant effect of co-infection onT. gondii(Pru) infection or replication in vitro. To further explore the factors causing the altered pathogenicity ofT. gondii(Pru) caused by co-infection, we found that decreased expression levels of IL-1β, IL-6, and IL-12 in the co-infected group were associated with the early immune responses againstT. gondii(Pru), which affected the division ofT. gondii(Pru). Moreover, the significant decrease in the CD4+/CD8+ratio indicated a weakened long-term immune killing ability of the host againstT. gondii(Pru) following IAV infection. In conclusion, aT. gondiitype II strain (Pru) could not be properly cleared by the host immune system after IAV infection, resulting in toxoplasmosis and even death in mice.