Because mouse mammary tumor virus (MMTV; the Bittner virus) is the proven cause of breast cancer in experimental mice, similar viruses have been suspected as a potential cause of human breast cancer. Recently, high-risk human papillomaviruses (HPVs), Epstein-Barr (EBV), and other viruses also have been identified in human breast tumors, but there is no definitive evidence for a causal role. There is a hypothesis that oncogenic viruses such as MMTV and high-risk HPVs may initiate some breast cancers. Recent studies suggest that EBV, HPV and MMTV gene sequences are present and co-exist in many human breast cancers, the presence of these viruses in breast cancer is associated with young age of diagnosis and possibly an increased grade of breast cancer.

All human carcinogenic viruses may also associate or cause breast cancer. This is also my recent interest dear Professor to read more about them, as many articles concerned with these viruses some proved this hypothesis, others suggest it. If you agree with me, may be this is one of the most common cause of breast cancer in our country(Yemen), providing the increasing incidence of this cancer among Yemeni females but this needs future studies to confirm this possibility. 

Here an article about( Role of viruses in the development of breast cancer):

http://www.infectagentscancer.com/content/8/1/32

I agree with you Dr Nohad that the role of viruses in most common cancers is certainly important, and we highly underestimated. Viruses can act as direct transforming agents and as triggering cofactors. The most probable mechanism of carcinogenesis may involve a combination of genetic alterations, immune system dysfunctions, and viral infections. I think further studies in our institute is needed.

Glenn WK et al says that EBV, HPV and MMTV gene sequences are present and co-exist in many human breast cancers, (ii) the presence of these viruses in breast cancer is associated with young age of diagnosis and possibly an increased grade of breast cancer.

for more information you can read this article

Epstein-Barr virus, human papillomavirus and mouse mammary tumour virus as multiple viruses in breast cancer

Mouse mammary tumor virus (MMTV) (Bittner virus), human papilloma virus (HPV) and Epstein-Barr virus (EBV) are the main viruses that induces breast cancer. Although very less cases have been reported by viruses.

I came across an "Breast cancer awareness program" organised by an NGO "Green Biology Research Centre" (GBRC) where they have mentioned the causes, symptoms and potential recommendations to get rid of the disease. They have also mentioned about the "Self Breast Exam" procedures. Find the attached file.   

The viruses associated with breast cancer include Mouse mammary tumour-like virus (MMTV), Bovine leukemia virus (BLV), Human papilloma virus (HPV) and

Epstein-Barr virus (EBV). Kindly go through the below given RG links.

Article Oncogenic viruses and breast cancer: Mouse mammary tumor vir...

Article Multiple oncogenic viruses are present in human breast tissu...

The viral cause of sporadic breast cancer (SBC) has been proposed based on an experimental mouse model of breast tumor induced by mouse mammary tumor virus (MMTV), Epstein-Barr virus (EBV), and human papillomavirus (HPV). While some studies have shown viral sequences of MMTV, HPV, and EBV in breast cancer cells, others have failed. Another study suggested that MMTV, HPV, and EBV have a potential role in the development of breast cancer and added more reasons to move forward in the search for a possible viral origin of breast cancer.

Kindly check the following RG link:

Article Human Mammary Tumor Virus, Human Papilloma Virus, and Epstei...