A human being is a union of vertebrate and virus
The human genome has evolved as a holobiontic union of vertebrate and virus... It is also probable that this "virolution" is continuing today. HIV belongs to a group of retroviruses called the lentiviruses. Until recently virologists thought that lentiviruses did not endogenise, but now we know that they have entered the germ lines of rabbits and the grey mouse lemur.
That suggests that HIV-1 might have the potential to enter the human germ line, perhaps taking our evolution in new and unexpected directions. It's a plague to us - but it could be vital to the biology our descendants.
Source : I, virus: Why you're only half human, January 27, 2010, Newscientist