CD4 and Opportunistic infection: The RP course has been suggested to represent a unique form of SIV disease, distinct from that of conventional progressors (CP) and associated with an unusual pathogenesis characterized by higher levels of SIV viremia, massive SIV replication in mononuclear phagocytic cells rather than in CD4+ T cells (with consequent lack of depletion of CD4+ T cells), and severe SIV-related enteropathy in the absence of opportunistic infections [34].