Endogenous Retroviruses and Evolution

 Endogenous retroviruses are viruses that after possibly being laterally translated into a genome, usually bacteria, are then passed vertically or spread through the reproduction of the host, the bacteria. It may confer immunity to other bacteriophages and remain passively translated into the bacteria's DNA. But, when the host under stress transcribes itself into viruses form (lateral), and breaks free, and searches for an alternate possibly a totally separate strain bacteria. Going from this current thread, if there was a viruses of this kind that could still activate in the genome of mammals, and specifically humans what would the results be? Transmission to a different host would not be nearly as simplistic, so the virus may be forced to form a kind of symbiosis with the host, performing some useful function  productive to both organisms, especially as always itself. My hypothesis in this hypothetical is that possibly the virus would work as an  to evolution of the host gathering viable genetic alternatives to better handle the environment that the host lives in. These possible alternatives could then be implemented with virus altering the DNA of the host's offspring while in the womb, with some form of backup, possibly continuous ovulation for a egg store.Trying out the alternatives on the offspring with some kind of trigger for a redo if the first experiment fails.

 While this is purely hypothetical, it would have fascinating implications for the theory of evolution. Nearly every species on earth would be affected if this was possible. It would allow the species to control its own evolution.