how breast cancer formed?

nobody really knows how it starts or how to prevent to have one. Some says its hereditary, some says exposure to carcinogenic materials/food/etc. But still no one could really dteremine where it comes from. The only prevention that you could do is perhaps have a clean living, maybe eating the right kind of foods and stay as healthy as possible and avoid getting depressed all the time since they say that stress could somehow trigger some cells to become cancer. Some says its important to have your immune system always at its best to prevent your resistance from diseases from infecting your body. Weak immune system could also lead to cancer since your body would allow cancer cells to multiply if your immune system is at its lowest point. So in short stay as healthy as possible to avoid developing cancer.