Every second, some 4 million cells in each of our bodies clone themselves by copying their DNA into two new cells, each virtually identical to its parent. The process isn't perfect, and occasionally a mutation occurs — if the process were perfect, we'd never have gotten beyond the original cell that kick-started life on Earth. Mutated cells usually die right away, but sometimes they survive. Countless reproductions later, their progeny become cancerous, forming a tumorous colony of "rogue" cells that can somehow evade the body's macrophages and other natural defenses. Not so difficult, since they're not exactly alien cells.

Researchers spent decades trying to figure out how many original mutated cells it takes to create a cancerous tumor. Thousands? Millions? The answer came in 2000, when researchers Douglas Hanahan and Robert Weinberg published "the single most influential paper on the biology of cancer," in the words of science journalist George Johnson. They dubbed their finding the "monoclonal theory," which is a giveaway: cancer starts with just a single mutation. This is why the disease is so common, and why only a fraction of cancers — perhaps a quarter, the jury's out — result from genetic, environmental or lifestyle causes. That is, for the most part, cancer strikes randomly. (Even smoking increases one's chances of getting lung cancer fractionally — the good news for smokers is that they still have "only" a 1 in 8 chance of dying from lung cancer, but the bad news is that they are 20 times more likely to die of the disease than non-smokers.)

Other than surgery, the two most common forms of cancer treatment are chemotherapy and radiation. Both work by interfering with cancer cells while they are dividing, the time when they are most vulnerable. Compared with most healthy cells, cancer cells divide very rapidly — think of it as "evolution while-you-wait." Unfortunately, some non-cancerous cells also reproduce quickly, including those in hair follicles, stomach lining and bone marrow (the source of immune cells). That's why chemo treatment (and to a lesser extent, radiation, which is more focused) causes baldness, nausea and immunosuppression.

The second most common form of cancer, after prostate, is breast cancer, the detection and treatment of which are particularly controversial. Do mammograms help or hinder? (False positives result in about 10 women receiving unnecessary and painful treatment for every life saved.) Is chemotherapy the best way to tackle it? (Probably not for post-menopausal women diagnosed with breast cancer, for whom chemo offers a paltry average of a 3 percent increase in their 10-year survival rate.)

The more we understand the disease, the more quixotic is the "War on Cancer" launched by President Nixon in 1971. Forty-plus years later, the war has not been won. Cancer is still the leading cause of death, with only a marginal drop in mortality rates since 1970, and it now appears unwinnable. No wonder. We live in an imperfect world where mutations are inevitable. Genetic errors such as those that cause cancer are part and parcel of life itself; to conquer cancer we'd have to conquer entropy!

Perhaps the best we can hope for in the future is a strategy of isolating tumors while maintaining them in a latent state. That is, learning to live with cancer.

Barry Evans (barryevans9@yahoo.com), who is 71, notes that 70 percent of men in their 70s who die of other causes already have prostate cancer.