As Agent Smith noted in the Matrix, transcribed by Next Big Future:
You move to an area and you multiply and multiply until every natural resource is consumed and the only way you can survive is to spread to another area. There is another organism on this planet that follows the same pattern. Do you know what it is? A virus.
Turns out he's right; according to Science Daily, about 8% of the human genome comes from viruses, and they have been there for millions of years.
Bornavirus in a cockatoo via Popular Science
University of Texas professor of biology Cédric Feschotte writes in Nature that the human genome contains DNA derived from bornaviruses. These are were discovered after they wiped out cavalry horses in the German town of Borna in 1885, and cause brain infections in birds and mammals. Their DNA was recently found in the human genome; Science Daily quotes Feschotte as suggesting that they might be responsible for psychiatric disorders such as schizophrenia.
Wired looks at the study by Keizo Tomonaga of Osaka University, which found the bornavirus genes. Tomonaga thinks the schizophrenia link is "not likely."
Because bornavirus genes insert randomly in the genome, "it is not conceivable that mutations [caused] by the integration lead to the specific brain disorders, such as schizophrenia," he says.