His interest in the emergence and spread of novel viral infections began in the late 1980s/early 1990s, as this was a time of the highest rates of HIV-associated deaths and when hepatitis C virus was first identified. He has spent over 30 years using molecular genetic techniques to understand the determinants of cross-species pathogen transmission and emergence, including SARS-CoV-2. His work has helped define the barriers faced by viruses as they emerge in new hosts, determine the range of transmission patterns exhibited by emerging viruses, and establish genetic models for host switching.