So why do people get cancer? That question sometimes comes up, in various permutations, at work. Or more often, patients attribute their cancer to something that an oncologist would not consider a likely cause. Like a middle-aged man with metastatic prostate cancer, who blames it on his summer job at a refinery back during college. Not likely.
If you surveyed the general public, I wouldn't be surprised if a majority of people considered pollution, or chemicals, or other man-made factors in our environment, to be the leading cause of cancer. And there may be some truth to that. For instance, the biggest environmental cause of cancer is probably cigarette smoke, which contributes not only to lung cancer but also bladder cancer (those chemicals have to go out somewhere), head and neck carcinoma, and a number of other malignancies. And yet, of people who smoke at least a pack a day - which is a lot more pollution than you'd get from your carpet cleaner, say, or sitting too close to the copy machine at work - less than 1 in 10 will get lung cancer.
The best description of cancer I've heard is that it's a "disease of the DNA." And when you consider the trillions of cells in our body, and the complexity of DNA and cell division, it's kind of a wonder that more people don't develop cancer more often. All it takes is one cell gone wrong, dividing uncontrollably, to start the ball rolling. And in fact, that's basically what cancer is: it almost invariably starts with just one bad cell.
So, tobacco smoke can damage DNA. So can radiation. And viruses can cause mutations that lead to cancer: most cases of cervical cancer are due to HPV, and the mononucleosis virus can contribute to some types of lymphoma, and chronic hepatitis B or C sometimes results in liver cancer. And definitely, there are other carcinogens out there, both man-made and natural. Sunlight, even, when consumed immoderately.
But mostly, I think of it this way: we live by our cells, and sometimes they go wrong on us. It's part of the price we pay for physiological sophistication developed over eons, as we evolved from simple one-celled things to fantastically complex organisms with eyeballs and emotions and innards (if you believe in that evolution stuff). Sometimes we can blame cigarettes, or faulty genes that have been passed through generations, or other factors. Aging! If a man lives to be 100, he is virtually guaranteed of developing prostate cancer. But the reality is that we can live our lives in the healthiest way conceivable, and certainly minimize our risks, but never eliminate it. Not while we're still dependent on cells ...