CD4 and AIDS: Clinically, the onset of AIDS is defined as the time point at which the CD4+ T-cell count in the blood falls below 200 per μL. The AIDS phase also often (but not always) coincides with a shift in the virus population and the emergence of virus strains that are able to use CXCR4 co-receptors (instead of CCR5 coreceptors) and thus a wider range of immune cells become susceptible to the virus [15,16].