As twists of fate go, this one is particularly twisted. It was announced over last weekend that Allen Carr, who gave up his 100-a-day habit 23 years ago and became one of the world's best-known anti-smoking campaigners, has been diagnosed with lung cancer.
"This has come as a shock but I remain very upbeat," the 73-year-old told a British Sunday newspaper.
Those who hope that quitting smoking will mean they avoid such diseases will probably not be feeling quite so defiant: They will want to know whether there is any point giving up at all.
The link between lung cancer and smoking was first made by Richard Doll in the British Medical Journal. On June 26, 1954, he launched a 50-year study, funded by the Medical Research Council, Cancer Research UK and the British Heart Foundation, of exactly how many years smokers were lopping off their lives.
On June 26, 2004, he announced that it was, on average, 10. However, he also discovered that quitting at age 50 -- the age Carr was when he stopped -- nearly halves that. Stopping at ages 60, 50, 40 or 30 gains, respectively, about three, six, nine or 10 years of life expectancy.
"It's obviously very sad that Allen Carr has now got lung cancer," says Amanda Sandford, research manager at Action on Smoking and Health, "but if he hadn't quit, he would probably have died years ago. He has certainly extended his life."
But it's not just a question of staying alive -- quality of life is also a factor.
"It's not just about death," says Robert West, director of tobacco studies at Cancer Research UK, "because obviously we've all got to die sometime. It's about disability. The reason people should stop smoking is not to stop them dying, but to stop them living a horrible, disabled life."
Smoking, like any addiction, consists in large part of the triumph of hope over observable fact. For example, it is comforting to believe that when you finally quit, the body will heal itself. A popular belief is that, by a certain length of time after quitting -- five years, say -- your lungs will be back to normal. Unfortunately, that is not the case.
"When you stop smoking, your lung cancer risk does not go down," West says. "What happens is that it stops going up. It's like a sort of escalator of death -- you want to try and get off on the lowest floor possible."
As well as writing Carr's Easy Way to Stop Smoking, which has been published in 45 countries, the give-it-up guru has established Easyway clinics in more than 30 countries.
His business partner, John Dicey, said at the weekend that Carr had "spent many years in smoke-filled rooms delivering his method to smokers; it is part of the clinic process that people actually smoke while there. He is not drawing any conclusions, but Allen feels that if that did contribute, it is a price worth paying, as we estimate he has cured around 10 million smokers."
West is unconvinced.
"It's more likely that he got it from his own smoking than from passive exposure," West says.
Carr was a very heavy smoker, for a long time, he points out.
"Lung cancer is a multi-stage disease -- it doesn't require just one event, but a succession of events eventually to become malignant and break through the body's immune system," he explains. "It's entirely possible that he sowed the seeds of this in early to middle age because if you stop at 50, you carry with you a greatly increased risk of lung cancer until the day you die."
The other major smoking-related killer is chronic obstructive pulmonary disease (COPD), a progressive and degenerative condition which eventually results in an inability to breathe; emphysema is a subset, as it were, of COPD.
In emphysema, the alveolar sacs which surround the alveoli merge into each other; they lose their elasticity and breathing in becomes more and more difficult. People don't often realize how affected they are -- 80 percent of COPD goes undiagnosed.
Sufferers simply assume that they are a little breathless, which comes with age, or perhaps lack of exercise -- and don't do anything about it. In fact, they are probably on a steep decline. By the time smokers reach 60, they can have a similar lung function to someone who has had asthma since childhood. Giving up smoking may halt that downward trend, but it can't reverse it.
"You never recover the lung function you have lost," West says.
After about 35, lung capacity and lung health decline anyway, so if they are already damaged, the decline is simply faster.
"People shouldn't be fooled into thinking that they can smoke for years and then, by stopping, remove all risk of smoking-related disease," Sandford says. "There will always be some residual risk."
It is true that not all smoking-related damage is irreversible. The risk of coronary heart disease drops by 50 percent in the first year after quitting smoking. Skin health also improves surprisingly fast.
"It is extraordinary how resilient the body is," West says. "It can put up with this insult day after day, year after year, and it forgives you -- until about the age of 35. After that, the clock is ticking, quite quickly. It's much more urgent than people realize."
All warn, however, that none of this counts as a reason not to stop smoking, and to do it as soon as possible.
"No matter how long you've smoked," Sandford says, "it's always worth quitting."