Environment/Heredity. Unfortunately, we don't know enough about breast cancer to be able to identify the specific causes; at best, we know of some conditions that are associated with breast cancer, such as high and/or prolonged unopposed estrogen levels (early menarche, late menopause, hormone replacement). App. 10-15% of cases are due to hereditary risks, the rest are environmental.
Many. The number one risk factor is being female. Other risk factors are family history, early menarche, late menopause, genetic predisposition, unopposed estrogen therapy, history of radiation to the chest wall, and a personal history of breast cancer.