To pass a viral alteration of DNA to offspring, the retrovirus has to infect germline cells, which they usually don't - and in a way that produces a viable offs

To pass a viral alteration of DNA to offspring, the retrovirus has to infect germline cells, which they usually don't - and in a way that produces a viable offspring, which is even rarer. That's not to say it doesn't happen over long enough time period - it clearly does. From wikipedia- https://en.wikipedia.org/wiki/Endogenous_retrovirus https://en.wikipedia.org/wiki/Endogenous_retrovirus Most retroviruses infect somatic cells, but occasional infection of germline cells (cells that produce eggs and sperm) can also occur. Rarely, retroviral integration may occur in a germline cell that goes on to develop into a viable organism. This organism will carry the inserted retroviral genome as an integral part of its own genome—an "endogenous" retrovirus (ERV) that may be inherited by its offspring as a novel allele.