Viruses are the most abundant life form and infect practically all organisms. Consequently, these obligate parasites are a major cause of human suffering and economic loss. The organization and origins of this enormous virosphere are profound open questions in biology. It has generally been considered that viruses infecting evolutionally widely separated organisms (e.g. bacteria and humans) are also distinct. However, recent research contradicts this picture. Structural analyses of virion architecture and coat protein topology have revealed unexpected similarities, not visible in sequence comparisons, suggesting a common origin for viruses that infect hosts residing in different domains of life (bacteria, archaea and eukarya).