CD4 and infection: It has been proposed that CD4+ T cell loss may be attributed to one of the following (1) Direct cell killing due to infection; (2) Apoptosis induced by viral proteins like the Env, Tat, Nef, Vpu, Vpr (reviewed by Gougeon 2005) [10]; (3) Cell death due to excessive activation of immune cells-activation induced cell death [11]; (4) Bystander apoptosis of neighboring uninfected cells [9].