Depending on how you do the sums (DNA sequences) and whether you consider 'junk DNA' as being of viral origin -- the human genes account for 1,5 per cent of the entire genome while the DNA derived from viruses amounts to 9 per cent.
Not only that, the major genes required for placental development are of viral origin -- retrovirus-like genes. Some are required for immunosuppression during pregnancy -- preventing the foetus from being attacked by the mother's immune system. For more, read the NewScientist article: Click here
