Retroviruses, in the norm, does not affect germ cells that makes sperm or egg therefore it's not passed down to offsprings. However, it does happen on rare occasion whete the retrovirus is passed down. A study done on Koala's showed a retrovirus that has been able to affect germ cells. KoRV-A is a retrovirus found in Koala's that makes them more prone to infection and cancer. What researchers found by looking at the passing down of KoRV-A is that when the retrovirus enters the germ cell they multiply and insert themselves into the host chromosomes, altering their genome organization and function. This showed the researchers that the genome immune system is able to tell a virus from their own gene. The system is able to make the decision to keep the genes the retrovirus has that benefits them and deemed the negative genes of the retrovirus as virus. Those categorize as a virus by the host immune system has virus-specific RNA, making them a target for germ cells that chops these RNA into smaller pieces called "sense" piRNA which then blocks the formation of the virus.
I thought this research on Koala would be very beneficial to study. By looking into how the Koala immune system is able to splice up the viral RNA, thus rendering them unable to replicate, can lead to treatments that involve virus related diseases.