  In people infected with HIV the RNA viral load is a good predictor of the
rate of loss of CD4 cells at a population level but there is still great
variability in the rate of decline of CD4 cells among individuals. Here we show
that the pre-infection distribution of CD4 cell counts and the distribution of
survival times together account for 87% of the variability in the observed rate
of decline of CD4 cells among individuals. The challenge is to understand the
variation in CD4 levels, among populations and individuals, and to establish
the determinants of survival of which viral load may be the most important.
