Diseases on new hosts -- A pressing issue in the present day world is the emergence of new diseases. While we have some answers about the emergence of specific diseases, such as hantavirus and AIDS, there have been almost no general study of this phenomenon in nature. Our studies consist of monitoring Microbotryum on Silene vulgaris as a model of an emerging disease, evolutionary analysis its origins and relationships using DNA sequence data, experimental studies of cross-species transmission, and post-emergence microevolution. In addition, by constructing species level phylogenies of both the host and pathogen and combining this with artificial inoculation experiments we hope to establish more rigorous methods for predicting disease emergence.