Introduction

HIV and other STIs spread via people's sexual connections across time and space. Transmission speed and trajectory through a population may be hastened or hindered by features of the underlying sexual network itself and the individuals who compose the network. Men who have sex with men (MSM) are disproportionately affected by STI.1 ,2 This disparity may be partly explained by three related concepts: (1) MSM networks are composed of individuals with higher behavioural risks (composition effects); (2) MSM network geometry predisposes MSM networks to higher levels of infection (network geometry) and (3) MSM demographic mixing patterns facilitate STI spread between groups (network topology). Here, we describe these three unique aspects of MSM networks and examine MSM sexual networks through a lens focused on two modern aspects of sexual partner recruitment—meeting partners during travel and through the internet and cellphone-based applications.