Cancer is a game of chance . It ' s not just simply damage to your DNA , it ' s damage to DNA in key places . There are proteins that check for damage to DNA and repair it , so your damaged DNA either needs to be too bad for that protein to fix , or the genes responsible for building that protein need to be damaged . There are proteins that detect irreparably damaged cells and force them to selfdestruct so they can ' t copy that damaged DNA , so the gene for that protein has to be destroyed . There are specialized immune cells that detect damaged , cancerous cells that aren ' t selfdestructing , and destroy them , so the gene that signals to outside cells that it needs to be destroyed has to fail . And of course , you have to damage DNA such that the cell reproduces when it isn ' t supposed to . That ' s a lot that has to go very wrong for cancer to happen . Every time you smoke , you ' re rolling the dice . You may damage a lot of DNA in a lot of cells , but you might be damaging the DNA that controls the growth of skin cells , but the DNA is in lung cells . So nothing happens . Or you might get rapid cell division , causing that cell to be cancerous , but it selfdestructs before it can cause problems , etc . Some people just get very lucky , the same way you might roll a die fifty times and never roll a 1 . Or you might roll the die three times and roll a 1 . There are some cellular mechanisms that biologists don ' t fully understand that prevent and control cancerous cells in our bodies . Some people for some reason are naturally resistant to cancer for those notunderstood reasons their cancer detecting proteins work better , or their cancereating immune cells work better . Or there are other factors , other risks that aren ' t directly related to the smoking that cause one person to get cancer sooner but that don ' t affect another person . There are a lot of things that increase your risk of cancer . Really , when someone says something causes cancer what it means is that those things make your risk of getting it go from a normal low background risk to You are almost certainly going to get cancer . But it ' s always risk , always different factors adding up that make your likelihood increase .