Transmission is the most important process underlying infectious disease dynamics [13] , but it is also the least understood. Rates of transmission are usually inferred from population patterns of disease incidence, but populationlevel analyses do not capture between-individual variation in transmission resulting from differences in behaviour, genetics, immune status, and environmental and stochastic factors, which play an important role in determining disease dynamics [14, 15] . Contact tracing has been used to directly measure case-to-case transmission, and applications of the technique to emerging infections such as SARS have generated important insights into disease transmission and control in human populations [16, 17] , but transmission processes for diseases circulating in animal populations are much harder to study.