<p>A recent review by Aguayo appearing in Infectious Agents and Cancer gave a timely overview of viruses that have been associated with human breast cancer. In this Matters Arising article, we discuss some critical additional points and concerns that are pertinent for this important and contentious research focus, in particular as it pertains to the involvement of the retrovirus mouse mammary tumor virus and its closely related human betaretrovirus form. This includes additional evidence for zoonotic as well as horizontal transfer of the virus and also odds ratio data on the likelihood of an involvement of the viruses proposed to be involved in human breast cancer. In our opinion, this information redresses the balance of the Aguayo review article and make a much more compelling case for this retrovirus being one of the causes of human breast cancer.</p><p><b>Clinical trial number</b> Not applicable.</p>