More than 20 percent of cancer deaths are caused by tobacco use. Next come obesity, lack of proper diet, lack of physical activity, and excess alcohol use. Other factors include infections, exposure to ionizing radiation, and pollutants in the environment.
Many cancers in the developing world are a result of infections like hepatitis-B and C, and human papillomavirus infections. Five to ten percent of cancers are a result of genetic defects inherited from one's parents.