CD4 and infection: While neutralizing antibodies are essential for the prevention of infection, the function of T cells (particularly, CD8 T cells and Th1 cells) is not to prevent infection but to control viral spread and limit viral replication within the infected host by direct lysis of virus-infected cells (CD8 and CD4 T cells) and through the production of antiviral cytokines such as IFN-gamma (CD8 T and Th1 CD4 cells) [18,19].