CD4 and infection: A single simulation for an individual in whom X4 emerges at approximately 4 years post-PHI is shown in Figure 4 A, B. Unlike the course of infection with only R5 tropic virus (Figure 3 A), here the CD4+ T cell counts exhibit an increased decline rate following emergence of X4 tropic virus (Figure 4 A), resulting in accelerated progression to AIDS [13], [15].