Although peripheral blood CD4+ T cells are resistant to infection in vitro[8]–[10], viral replication occurs predominantly in lymphoid and mucosal tissues, where microenvironmental factors, including cytokines such as IL-4 and IL-7 maintain resting T cell viability [11]–[17], and fortuitously (for the virus), facilitate their productive infection [18]–[22]. The gene discussed is CD4; the disease is infection.