Infectious diseases affecting humans, agriculture and wildlife often result from pathogens (virus, bacteria and parasite) shifting from one host species to another. It is therefore critical to measure the propensity of a pathogen to shift host. Researchers from Lausanne and Stanford used host-parasite associations, species’ evolutionary histories and computational simulations to provide a new method to estimate host shift rate and expected time to the next host shift. With their method, they provide a reliable estimate of host shift for the avian malaria parasites and the chewing lice of pocket gophers. This research provides new avenues for quantifying host shift rates of pathogens. More importantly, it also allows for the prediction of the rate of possible host shifts and thus, potentially, the emergence of novel infectious diseases.