“VIRAL PARTICLES are much more abundant than cells and viral genes outnumber cellular ones in the biosphere. Cellular genomes also harbour many integrated viruses whereas cellular genes are rare in viral genomes. The gene flux from virus to cell is thus overwhelming if compared with the opposite event. Cells are giant pickpockets of viral genes and can mimic viral biology.”
http://www.sciencedirect.com/science/article/pii/S1879625713001077