For most of the common cancers, a major cause has been identified: Smoking causes 90 percent of lung cancer worldwide; hepatitis viruses cause most liver cancer; H pylori bacteria causes stomach cancer; human papillomavirus causes almost all cases of cervical cancer; colon cancer is largely explained by physical activity, diet, and family history.
But for breast cancer, there is no smoking gun. It is almost unique among the common cancers of the world in that there is not a known major cause; there is no consensus among experts that proof of a major cause has been identified.
