‘Viruses are everywhere’ describes the ‘virosphere’—the huge virus biomass of enormous variety and complexity in the environment. Viruses are the most numerous microbes on Earth with an estimated 100 million different types. They are also the most abundant life forms in the oceans. Marine viruses kill an estimated 20–40 per cent of marine bacteria every day, and as the major killer of marine microbes, they profoundly affect the carbon cycle by the so-called ‘viral shunt’. Viruses are also important to land-based ecosystems. Despite their tiny size, viruses are proving to be of prime importance in the stability of ecosystems worldwide. The possibility that viruses exist in outer space is also considered.