Leigh Johnson is an epidemiologist and actuary, working at the Centre for Infectious Disease Epidemiology and Research (University of Cape Town). His research interests are in modelling of HIV and other infectious diseases, and most of his published work has focused on evaluating the impact of HIV prevention and treatment programmes in South Africa. He is the lead developer of the Thembisa model, a combined HIV and demographic model developed for South Africa (see www.thembisa.org). He is a member of the International Epidemiology Databases to Evaluate AIDS (IeDEA) collaboration and the UNAIDS Reference Group on Estimates, Modelling and Projections. He is also an editor of the journals AIDS and Infectious Disease Modelling.