Viruses are noncellular forms of life and are much smaller and less biochemically complex than the simplest unicellular organisms. They consist of either RNA or DNA as a single molecule, or in some cases as a segmented genome, enclosed by one or more proteins. These proteins protect the nucleic acid from degradation; deliver it to the host cells that reproduce the virus; transcribe the nucleic acid (in the case of negative stranded genomes); and assist the virus to expose the nucleic acid to the biochemical machinery inside susceptible host cells. This relative simplicity has in part been the secret of the success of viruses in coexisting with all known life forms.
