Viruses have long been held to be derailed cellular elements with a parasitic replication cycle: since without a cell there is no virus. However, the explosion in recently acquired molecular data from genome and protein sequences leads to a completely different view: viruses contain sequences which are unique in the biosphere, which is difficult to combine with the idea of cellular origin. Conversely, some cellular sequences appear to be of viral origin, such as the development of DNA as the most important carrier of genetic information. Recent investigations indicate that viruses probably represent a crucial step in the evolution of the cell. This idea is a great step forwards in understanding the evolution of the cell and explains why viruses have continued to exist alongside cell based lifeforms. Viruses, which are often considered only as enemies of their host, therefore ultimately constitute a driving force in evolution.