Breast cancer affects one in eight women during their lives. Breast cancer kills more women in the United States than any cancer except lung cancer. No one knows why some women get breast cancer, but there a number of risk factors. Risks that you cannot change include
Other risks include being overweight, using hormone replacement therapy, taking birth control pills, drinking alcohol, not having children or having your first child after age 35 or having dense breasts.
Men can have breast cancer, too, but the number of cases is small.
MedlinePlus - Breast Cancer