CD4 and infection: However, for progressor cat 184, although CD4 T cells profoundly decreased from ~200 weeks post-infection (PI) onward, CD8 T cells initially decreased from approximately 340 to 425 weeks PI and then returned to the normal range (500–2500 cells/μL blood) from 450 weeks PI until euthanasia at 12 years of age (Figure 3a).