Let me explain. The sort of person who habitually wakes to a nagging hangover, and then opens his nicotine-raddled lungs with a coughing fit, barely notices a mild cold or a bout of flu: he feels his usual grotty self, only more so (I know because I have tried the lifestyle). But when your weekly regime revolves around regular doses of physical exertion - a couple of runs in the park, perhaps a game of squash, a game of football and an hour refereeing the under-eights on a Saturday morning - your system becomes addicted to the release of adrenaline. Which means that you feel wretched from the cold itself and then stale, irritable, claustrophobic and headachey from the lack of exercise: a double-whammy of woe.
Or was this my excuse for my appalling temper last week, much of which I spent struggling against that first cold of the winter. For a few days I tried carrying on as normal, hoping to face down the blockages - a tactic that sometimes works. I fuelled myself with Lemsip, lost even more abjectly than usual on the squash court, and tried to avoid blowing my whistle when refereeing for fear of filling it with phlegm. This time it didn't work; first, physical activity became impossible, then concentration. At which point I caved in and took to my bed for a day, and slept long enough to shake off the worst of it.
But I remain convinced that some exercise can help prevent these sorts of minor illnesses - and, in all probability, various major ones as well. Those hardy types who swim out of doors all the year round, and have their 10 seconds of fame on Christmas Day when they plunge into the Serpentine or the North Sea in front of the television cameras, claim they never catch colds or flu. I believe them.
The only trouble is, I discovered the hard way that I couldn't join them. One autumn a few years ago, when I had access to one of the few outdoor pools that stay open through the winter, I vowed to swim every day in my lunch break. Surely, I reasoned, the water would not seem cold if I swam so often that I acclimatised myself to the falling temperature as it dropped by small fractions of a degree each time.
All went fine until late November. Although it took a little longer to warm up each day, I knew that I would soon become accustomed to the cold water, even though it was now under 60 degrees, well below anything I could normally stand. Friends were amazed that I was still swimming out of doors, while I felt smugly healthy and congratulated myself on the accuracy of my theory.
Then one day disaster struck. I dived in, felt the habitual impact of the cold, and swam a couple of quick strokes while waiting for the initial shock to wear off, as it always did. It didn't. Instead, it transformed into an intense, cold pain that seemed to squeeze the very breath from my body, and culminated in a piercing headache. I swam in short, snatched strokes to the shallow end, breathing with difficulty, dragged myself out of the water and lay there exhausted after barely a minute in the pool.
The temperature cannot have been more than half a degree below what my body had found acceptable, if not comfortable, the day before. I concluded that each of us must have a pre-set physical endurance limit - like the altitude limit mountaineers discover high in the Himalayas - below which we cannot operate.
It did occur to me to try the experiment again, wearing a wetsuit, but would you still enjoy the protection from colds and flu? Probably not. And frankly I've now come down on the side of the vast majority of the population who feel that a week or so of flu every year is a small price to pay to avoid the daily trauma of diving into dangerously cold water.