The understanding of how viruses affect all life on earth has exploded in recent years. For example, it has become clear that viruses in the ocean outnumber all other organisms and that their influence on marine communities may have large-scale effects on the composition of the atmosphere. Viruses are also ancient agents of selection for evolutionary adaptation. A lively debate on viral origins argues that they predate the Last Universal Cellular Ancestor (LUCA) and may be responsible for driving the change from an RNA to a DNA world . Viral disease played a critical role in shaping key events in human history—for example, smallpox epidemics killed over 300 million people in the last 700 years, including a large number of heads of state, and allowed the European conquest of the Americas. Emerging and re-emerging viral pathogens, from SARS to avian influenza to foot-and-mouth disease, continue to adversely affect our health and the viability of our food supply. Finally, a large percentage of our own DNA is actually made up of fossils of ancient viral infections that got “stuck” in our genome; so, in a sense, we are descendants of viruses as much as we are of our human ancestors.