Surveillance—the routine, systematic collection of data about disease frequency and distribution; the analysis of those data; and the dissemination of that information to those who need to know—is a cornerstone of public health action.1 Surveillance has played an important historical role in the global response to HIV/AIDS, alerting the world to the gravity of the pandemic, providing data on which major funding decisions were based, allowing appropriate targeting of intervention efforts, and evaluating and monitoring the response. Despite the complexity of measuring HIV infection, morbidity, and mortality, data on the global distribution of HIV/AIDS are among the most robust in all of global health.

Traditionally, estimates of national and global burdens of HIV infection have been derived from mathematical modeling applied to sentinel HIV prevalence data among pregnant women.2 Early on, sentinel studies among persons with sexually transmitted infections and blood donors contributed additional information. So-called “second generation surveillance” introduced in the early 2000s emphasized adapting approaches to local country contexts, addressing the surveillance needs of key populations at high risk of exposure to HIV infection, collecting information to monitor and measure the impact of HIV services, and incorporating appropriate behavioral measurements to give integrated insight into biomedical data.2 Despite these improvements, increasing experience with population-based surveys showed that extrapolations from sentinel surveillance data frequently yielded excessively high estimates of population HIV prevalence, and the role and utility of these national HIV prevalence surveys quickly became recognized.3 The substantial downsizing of global estimates of HIV infection that the Joint United Nations Programme on HIV/AIDS (UNAIDS) and the World Health Organization (WHO) undertook in 2007 can largely be ascribed to the insights provided by population-based surveys in different countries.4