There are a lot of risk factors for Cancer
1)Smoking can cause Lung Cancer
2)Exposure to ionizing radiation can cause some forms of cancer
3)Some cancers can be caused by viruses, for example the Human Paploma Virus(HPV)* can lead to Cervical Cancer.
4)Exposure to coal dust can lead to lung cancer, this tends to hit miners and the like.
5)Some chemicals can lead to cancer
Mind you none of these things are 100%, but all can increase the risks. Best advice is to not smoke.
*There is now a Vaccine for HPV