‘Viruses are everywhere’ describes the ‘virosphere’ — the 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. The largest virus, mimivirus, is thought to fall into an evolutionary position at a point before the animal and plant kingdoms split and indicates the long history of viruses. The abundance of viruses in the oceans means they control the dynamics of marine populations, their interactions, and the entire marine ecosystem. Viruses are also important to land-based ecosystems and their reproductive abilities are highlighted when considering cholera epidemics.