Transfer of Icos−/− LCMV-specific SMARTA CD4+ T cells into wild-type mice followed by LCMV infection resulted in a marked defect in Bcl6 and CXCR5 expression at days 3 and 4 post-infection relative to wild-type SMARTA T cells, suggesting that ICOS plays an essential role in the induction of Tfh cells. The gene discussed is BCL6; the disease is infection.