To answer this question, it is necessary to introduce an important theory in oncology – the “second strike doctrine of tumor”, that is, all tumors must have two inevitable conditions to arise, one is the congenital genetic condition; the other is the influence of the acquired adverse factors. One of these factors alone will not lead to tumors, but only the synergistic effect of genes and acquired factors will lead to tumors. Generally speaking, if a person’s genetic condition is good enough, even if the external environment is bad, the person will not get tumor; but if a person’s genetic condition is too bad, no matter how careful he is, he will still get tumor. Most of us have genetic conditions between these two kinds of people – if we are careful to prevent, we will not get tumor, if we are not careful to expose to or even long-term exposure to tumor-causing factors, we will develop tumor. We do know that some smokers and alcoholics do not get tumors, such as the famous Stalin and Churchill, but we can only say that these two people were born with too good genetic conditions, while we in general should be careful to lead a healthy life. Why do some people find lung cancer soon after they quit smoking? First, quitting smoking does reduce the incidence of lung cancer, but the decline is very slow – the risk of lung cancer is reduced by 50% after 5 years of stopping smoking, and the incidence of lung cancer only drops to a level similar to that of nonsmokers after 15 years of quitting. Second, the development of tumors is a long and slow process, from the time a single cell is stimulated by an external adverse factor to produce a mutation to the time it forms a tumor large enough to be detected generally takes 5 years or more, so some research theories say that the tumors found now are the result of adverse factors stimulated 5 years ago. Therefore, it is not the fault of quitting smoking to get lung cancer again after quitting. If you do not quit smoking, you will also get lung cancer, and it may even happen earlier.