Viruses are not cellular and are therefore described as "biological entities" rather than organisms. They grow as intracellular parasites after invading cells. Viruses usually consist of one molecule of nucleic acid (either DNA or RNA) wrapped in an envelope made largely or completely of protein. The envelope is specialized to allow the virus particle to enter particular plant, animal, or bacterial cells.

Most viruses contain virtually no metabolic machinery of their own, so they use that of the host cell to reproduce. Viruses thus provide useful insights into the cellular functions that they co-opt during infection.

For example, the smallest DNA-containing viruses replicate their DNA using only host-cell enzymes. Because of their small size, these viral DNA molecules can be isolated and characterized much more easily than the giant DNA molecules in cellular chromosomes.