Why should you care whether or not a virus causes breast cancer?
What difference would it make if it were true?
not a bioinformatic question: ask here : http://biology.stackexchange.com/
how is your question related to bioinformatics ? you'd better ask http://biology.stackexchange.com
I agree with the with comments that this is off topic. But for future reference ... if breast cancer was caused by a virus, and we could develop a vaccine for said virus, we could cure breast cancer. That's why we would care...