All infectious disease epidemics have similar stages: an early epidemic stage when R0 (the reproductive number of the infectious agent) is the highest; a slowing of epidemic spread when R0 begins to decrease; a peaking or leveling of epidemic transmission; and finally a decreasing phase when R0 is < 1. The impact of HIV prevention programs on any national prevalence trend is very difficult to measure since there are no valid control populations for comparison. If HIV was truly an infectious disease agent for which all or most persons in the population were at about equal risk, then significant differences in HIV prevalence in one population compared with another might be attributed to differences in prevention programs.

If prevention programs are implemented when HIV epidemics are at or near their “natural” peaks, the subsequent decrease in prevalence might be incorrectly attributed to prevention programs. Yet most of the observed decrease might more likely be due to a saturation effect – infection of most of the population with the highest risk behaviors. Thus, only a small proportion of the decrease in HIV prevalence might be due to prevention measures.*

* Dr. Alex Langmuir, the father of the Epidemic Intelligence Service (EIS) program at CDC, Atlanta, referred to this epidemiologic situation as “riding to glory on the down slope of the epidemic curve.”