As a composite, these data show that type I IFNs suppress the development or accumulation of Tfh cells and promote the expansion of T-bet+Blimp-1+ effector CD4 T cells during blood stage Plasmodium infection and that the IFNα/β-Blimp-1 circuit is a generalizable feature of systemic, Th1-biased infections. This evidence concerns the gene IFNA1 and infection.