The epidemiologic concept of the reproductive number of an infectious disease agent (R0) is very specific and is more useful to describe HIV transmission dynamics, especially the difference between epidemic and nonepidemic HIV transmission. R0 describes in a single value, the epidemic potential of an infectious agent. When, on average, one infected person infects more than one other person, R0 is >1 and the result will be epidemic spread of the agent. When, on average, one infected person does not infect more than one other person, R0 is <1 and epidemic spread does not occur. When R0 is <1, the infectious agent will slowly disappear and if R0 stays close to 1 the agent will maintain itself in the population with no or minimal growth (i.e., becomes endemic).