Our data suggest that rates of genetic exchange can vary by orders of magnitude across viral taxa; although reassortment seems rather uncommon in many viruses, within the Cystoviridae, we find it to be commonplace. However, previous studies did not take a quantitative approach to examining rates of genetic exchange in wild populations of viruses. Therefore, our data should motivate greater effort to determine whether other viruses frequently undergo sex, which may cause them to feature a population structure that is truly panmictic. Mutation has long been appreciated for its role in promoting genetic variation in viruses (especially in those with RNA genomes) (44) and implicated as a key force in disease-related processes, such as the crossing of host-species boundaries (48, 51). But a dearth of studies that accurately measure rates of recombination or reassortment in natural populations might have caused the role of genetic exchange to be underappreciated. Our discovery that natural populations of highly diverged microbes can feature extremely low levels of LD is unprecedented.