Leigh is an epidemiologist and actuary, with a particular interest in the modelling of HIV and other diseases. He leads the development of the Thembisa model, an integrated HIV, TB and demographic model for South Africa. He also leads the development of MicroCOSM, an agent-based model of the social determinants of HIV and other STIs in South Africa. He is a co-chair of the UNAIDS Reference Group on Estimates, Modelling and Projections, and is a member of the editorial boards of ‘Infectious Disease Modelling’ and ‘Journal of the International AIDS Society’.