Although there is no geological record of viruses, the analysis of the relationship between the amino acid sequences of viral and cellular proteins and that of the nucleotide sequences of the genes encoding them provide ample genetic evidence that the association between viruses and their hosts is as ancient as the origin of the hosts themselves. Some viruses (e.g., retroviruses) integrate their genetic material into the cell they infect, and if this cell happens to be a germ line, the viral genome (or its relict) can be maintained essentially forever. Analysis of the sequence relationship between various retroviruses found in mammalian genomes demonstrates integration of some types before major groups of mammals diverged.
While the geological record cannot provide evidence of when or how viruses originated, genetics offers some important clues. First, the vast majority of viruses do not encode genes for ribosomal proteins or genetic evidence of relicts of such genes. Second, this same vast majority of viruses do not contain genetic evidence of ever having encoded enzymes involved in energy metabolism. This is convincing evidence that the viruses currently investigated did not evolve from free-living organisms. This finding distinctly contrasts with two eukaryotic organelles, the mitochondrion and the chloroplast, known to be derived from free-living organisms, and is convincing evidence that the viruses currently investigated did not evolve from free-living organisms.
Genetics also demonstrates that a large number of virus-encoded enzymes and proteins have a common origin with cellular ones of similar or related function. For example, many viruses containing DNA as their genetic material and viral-encoded DNA polymerase are clearly related to all other DNA polymerase isolated from plants, animals, or bacteria. The reverse transcriptase enzyme encoded by retroviruses, and absolutely required for converting genetic information contained in RNA to DNA, is related to an important eukaryotic enzyme involved in reduplicating the telomeric ends of chromosomes upon cell division.
Viruses are very tiny organisms with their diameters in nanometers level. Viruses exist almost everywhere, such like in the air, soil and animal body. Human body also habors a lot of viruses, but most of them do not cause diseases. Human immune system can generate antibodies against viruses, the antibody will recognize the virus and destroy it.