Viral counts count in HIV infection.

When the acquired immunodeficiency syndrome (AIDS) first appeared, its pathogenesis was frustratingly elusive because the disease does not appear immediately upon infection with the human immunodeficiency virus (HIV). There is a variable period of time during which the patient remains healthy but exhibits viremia (the virus can be detected in the patient’s blood). Recent work shows that this viremia is sustained by continuous rounds of viral replication and reinfection of blood cells (1, 2), dispelling the notion that HIV has a true latent period. It follows that measuring the amount of virus in the blood (viral load) should be useful in determining the prognosis of the infected individual and in monitoring the effectiveness of therapies. Among a number of reports in support of this simple view is the elegant study in this issue by Mellors et al. (3), who show that a single measurement of plasma viral load can predict the subsequent risk of AIDS or death.