In this study, we found that a larger number and proportion of CD4+T cells were produced in the spleen of B. microti-infected tumor-bearing mice for clearance of infection, suggesting that B. microti infection may activate the immune system of tumor-bearing mice and cause the body to produce large amounts of CD4+T cells, thereby inhibiting tumor growth (Nishimura et al., 1999). This evidence concerns the gene CD4 and infection.