There has been considerable progress made in the study of viruses and how they infect, replicate and spread inside hosts and host cells. Much is known about how viruses change their genome and proteins throughout their life cycles. However, our understanding of how the host cell itself changes during the virus life cycle is less well developed. Both genomic and proteomic changes are expected to occur during virus propagation, and genomic analyses of the cellular 'transcriptome' have been especially fruitful during the past decade. However, our understanding of host protein responses is even more poorly developed. These âomic responses can reflect particular host metabolic pathways, such as immunological strategies, that are activated in response to the infecting virus. These genomic and proteomic changes also may suggest ways in which the infecting virus alters the cell components to aid its propagation. By studying not only how the virus changes during infection, but how the host responds, one might determine which of these responses are important for viral survival, and exploit them for antiviral purposes.