Against the background of human immunodeficiency virus (HIV) and acquired i
mmune deficiency syndrome (AIDS) and other potentially emerging (or re-emer
ging) infectious diseases, this review will focus on the properties which e
nable an infectious agent to establish and maintain itself within a specifi
ed host population. We shall emphasize that for a pathogen to cross a speci
es barrier is one thing, but for it successfully to maintain itself in the
new population is must have a 'basic reproductive number: R-0, which satisf
ies R-0 > 1. We shall further discuss how behavioural factors interweave wi
th the basic biology of the production of transmission stages by the pathog
en, all subject to possible secular changes, to determine the magnitude of
R-0. Although primarily focusing on HIV and AIDS, we shall review wider asp
ects of these questions.