‘Viruses’ explains that millions of people die from interacting with viruses every year, but beyond the effects of viruses on human health, the lives of all organisms and the cycling of nutrients through the biosphere depend upon the activities of viruses. Viruses control populations of bacteria, archaea, and eukaryotes and this destructive power liberates massive quantities of nutrients in aquatic and terrestrial ecosystems. Viruses are organized into seven groups based upon the type of genome and its mechanism of replication. Viral genomes are encoded in single-stranded and double-stranded DNA and RNA molecules. The expression of viral genes occurs within infected cells utilizing the molecular machinery of the host.