"At present it is still scientifically unclear exactly how CMV might cause or influence the development of breast cancer."
All cancer develop in the same way. The connection to inflammation. Cancer is a viral disease of the stem cells.
Virus infects an organ. A strong host will setup an acute inflammation and wipe up the virus quickly. A weak one will tolerate a chronic inflammation for long time. Inside the cell
there are at least two possibilities: either the virus kills the cell and produces more virons or parts of it are incorporated in its DNA and the cell is
protected from similar viruses. When stem cells are hit, the organism triggers another protect mechanism, namely tumor. The adult organism lost its ability to generate stem cells. Stem cells are assumed (see links below), among many other
tasks to secrete a substance that prevents cachexia. For more info please see on you-tube a presentations by Gershom Zajicek M.D, Cancer is a Triad [Tumor, Para-neoplasia, Cachexia], A New Cancer Hypothesis Update 2015 and many others.