ABSTRACT
The epidemiologic concept of the reproductive number of an infectious disease agent is very specific and is more useful to describe human immunodeficiency virus (HIV) transmission dynamics, especially the difference between epidemic and nonepidemic HIV transmission. A major factor that makes the epidemiology and transmission dynamics of HIV different from most other sexually transmitted infection (STI) is the generally very low infectivity of HIV via sexual intercourse. A specific example was the importation in the mid-1980s of HIV-infected blood products for the treatment of thousands of male hemophilia patients in Japan. A high level of condom use can have a major effect in reducing HIV transmission. Sexual mixing patterns and the frequency of sex partner exchanges are the primary determinants of the rapidity and extent of epidemic sexual HIV transmission. The patterns and prevalence of heterosexual risk behaviors are not the same in all the world's populations.
