The need to prevent and cure emerging diseases often precludes their continuing study in situ. We present studies on the process of disease emergence by host shifts using the model system of anther-smut disease (Microbotryum violaceum) on the plant genus Silene (Caryophyllaceae). This system has little direct social impact, and it is readily amenable to experimental manipulation. Our microevolutionary studies have focused on the host shift of Microbotryum from Silene alba (=latifolia; white campion) onto Silene vulgaris (bladder campion) in a population in Virginia. Karyotypic variation shows that the host shift is recent and originates from the disease on sympatric S. alba. Analysis of the spatial pattern of disease shows that the host shift has been contingent on the co-occurrence of the two species at a local scale. Cross-inoculation studies show that families of the new host differ greatly in their susceptibility to the pathogen, indicating the potential for rapid evolution of resistance. Disease expression on the new host is frequently abnormal, suggesting that the pathogen is imperfectly adapted to its new host. In experimental populations, disease transmission within populations of the old host is greater than within populations of the new host. However, there is also a high transmission rate of the disease from the new host back to the old host, suggesting a feedback effect that increases disease prevalence in the community as a whole. Continuing studies of these populations are designed to determine whether this new host-pathogen system is likely to be self-sustaining and to quantify evolutionary changes in both the host and the pathogen.