HIV’s replication capacity (RC), a measurement of the virus’s fitness, may be useful for people living with HIV and their health care providers in figuring out how quickly HIV disease will progress, according to new data published in the April 1 issue of the Journal of Acquired Immune Deficiency Syndromes (JAIDS). The study results show that untreated people living with HIV with lower, compared with higher, RC had a slower progression to a CD4 cell count below 350 cells—the widely accepted threshold for starting antiretroviral (ARV) therapy.