For anyone who still believes the only retroviruses infecting humans are HIV and HTLV, I would like to reference this Berkeley announcement and this published paper. This follows a relatively unnoticed publication last year showing bovine leukemia virus (BLV) could be found in humans. Since antibodies to BLV are found in essentially all milk tankers, and in people who drink unpasteurized milk, the reason for saying the virus was not found in humans, based on old studies with very low sensitivity, is hard to square with virtually all work on retroviruses in non-human animals. If the virus is passed from mother to infant prior to weaning, and the effects do not appear until after sexual maturity, we have to expect the rate of replication to be inversely proportional to the time between weaning and sexual maturity, which is unusually long in humans. A virus which replicates faster would often kill infected individuals before they could pass it on, likely becoming extinct. We don't know that the virus is the cause of this human disease, though similar viruses cause virtually all mammary tumors in some other mammals. Also, patients on ARVs are known to have lower rates of breast cancer. There is a possibility the association takes place after onset. This still leaves the question of markers associated with both infection and onset which might lead to early detection --- when the problem is easy to treat. Early detection has long been a research goal. The size of the reported increase in risk is larger than most other known factors. The effects of this disease are undeniably serious, and the number of cases is substantial. So why has this possibility, and the chance to reduce incidence of breast cancer, met with such strong opposition over a period of many years? In cattle the disease most obviously associated with BLV is bovine leukosis, but many cattle tolerate infection without displaying symptoms. Leukocytes are the cells most affected, and many diseases associated with impaired immune response could be the result of retroviral infection of leukocytes.