I wrote a long post about bovine leukemia virus here:
Bovine leukemia virus — a retrovirus found in around 33% of people: could it play a role in ME/CFS?
This virus is known to be immunosuppressive in cattle, so I wondered if it might be immunosuppressive in people. Since at least a third of the human population is infected with bovine leukemia virus, any immunosuppressive effects may be a significant health consideration.
I think the jury is still out regarding whether bovine leukemia virus can cause human breast cancer, but it's certainly a possibility.