Infectious disease dynamics: what characterizes a successful invader?

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.