Emerging diseases are often the product of a host shift, which occurs when a parasite jumps from its original host and into a novel host species. Shifts in host range have caused major epidemics in humans and other animals, such as HIV, the Spanish flu, and Ebola. Our research group identified the original host ranges of bacteriophages initially isolated on a single host, Escherichia coli strain C. We found that these well-characterized viruses are all of the same family of Microviridae, but that they differ vastly in the number and type of bacterial species they can successfully infect. Through a series of gene swap experiments, we successfully constructed hybrid bacteriophages from a generalist and a specialist virus. We aim to determine which specific genes are responsible for these variable phenotypes. Insights from these studies regarding the evolution and genomics of host range will aid our understanding of viral emergence. By exploring the evolutionary mechanisms underlying changes in host range, we could better respond to, and even prevent, future shifts of disease-causing viruses.