CD4 and infection: NK and NKT cells are the sources of IFN-γ during the early periods of infection and are followed by CD4+ and CD8+ T cells, which are the main producers at the peak of host response to infection; 2) production of IL-12 and IL-18 precedes IFN-γ and optimal IFN-γ production relies on the combined action of IL-12 and IL-18; and 3) IFN-γ is essential for NOS2 induction and NOS2 plays an important role in controlling virus replication.