Out of all the diseases that affect human beings, cancer is particularly hard to grasp. After all, bacteria and viruses had millions of years to evolve into deadly attackers. Cancer cells are just the product of a random cell gone haywire. So how can they possibly account for the second most likely cause of death in the industrialized world?
The answer is bad luck and big numbers. Although cancer-causing mutations are exceedingly rare (on an individual-cell basis), your body has trillions of cells, and it manufacturers millions more every minute. Even a seemingly miniscule rate of cancerous mutation—say, one in a million cell divisions—would guarantee you a terminal case of cancer.
The real question isn’t why we get cancer, but why we don’t get it more often. In fact, the body has countless built-in safety measures to defend against cancerous cells. It has specialized genes that detect suspicious behavior and shut down cell growth or trigger cell destruction. Your immune system even has a class of specialized warriors called natural killer cells that hunt down cancerous cells and release toxic granules that destroy them.
However, none of these mechanisms is completely foolproof, and given enough time (and enough cell mutations), a cancerous cell can start to thrive. Cancer is a particular problem in old age because the body’s cells have had more time to accumulate the right mutations, the natural cancer fighting processes of the body have weakened, and a lifetime of exposure to carcinogens (chemicals in the environment that can damage DNA and spur mutations) has taken its toll. The following chart shows the rate of colon cancer as a function of age, and it tells a clear story—cancer risks skyrocket as time wears on.
It’s all a bit ironic. In one context, sheer mind-boggling odds can turn one mutation in a trillion into the survival advantage that drives the evolution of a species. But in another, a chance combination of deadly mutations can trigger a cancer and destroy an individual.
Source of Information : Oreilly - Your Body Missing Manual