Today marks that particularly peculiar date in the British calendar. It's a day when forlorn hope suddenly takes over from reason.

Shut your eyes and you can smell it. It's the day that heralds the outbreak of "Henmania".

Get within five miles of London SW19 and the aroma is overpowering. Temperatures rise and fever is in the air. Shakespeare had the phrase "Midsummer madness". In his day, people believed that dogs were more susceptible to rabies when summer was at its height.

But what would the Elizabethans have made of people who, year after year, at exactly the same time, in exactly the same place, delude themselves that a Brit is going to win Wimbledon? In an hysterical age, Henmania is one of the oddest hysterias of the lot.

It is not like Beckham mania, which is a year-round phenomenon, afflicting young and old alike, ravaging whole continents, causing fainting fits from Madrid to Tokyo.

Henmania is intense while it lasts, then quickly subsides - until 12 months later, when it flares up again.

Men in suits chanting "Timbo! Timbo!" as if their lives depended on it. Women in Laura Ashley dresses brandishing "ANOTHER ACE FOR TIM" placards. Some idiot losing it completely and streaking across the Centre Court.

Another year, the same old Henmania.

Its focus is not a sporting titan, endowed with the charisma of the breed. On the contrary, he is a plain, clean-cut former public schoolboy whom you might pass in the street without taking a second look.

When Tim Henman competes in the other Grand Slam tournaments, the nation takes no notice. How many of us remember at which stage he was eliminated from the Australian or French Opens earlier this year? Or which faceless foreigner, ranked umpteenth in the world, knocked him out?

But British tennis fans must be grateful for small mercies, so let nobody decry what Tim Henman has achieved at Wimbledon. After years in which British interest in the tournament fizzled out in the first week, we have come to expect a Briton to reach the last eight or even the last four - expectations which Henman has met with admirable consistency.

He has been a semi-finalist four times and, in 2001, but for an untimely rain interruption in his match against Goran Ivanisevic, would probably have gone one better.

But he is never - barring miracles - going to win the tournament. You had only to watch his exit in the semi-final last year, beaten by a younger, tougher, hungrier competitor, in the shape of Lleyton Hewitt, to realise that his moment had passed.

This year, he is nursing a shoulder injury and has been out of form. Next year he will be nearly

30. Time waits for no man.

History will judge him as one of the nearly-men of sport: someone with the technical ability to get to the very top, but without the steel in the soul that characterises the true winner.

The great British sporting champions have had a driven, awkward, bloody-minded quality. Look at Nick Faldo. Look at Ian Botham. Look at Steve Redgrave.

Henman belongs in the second rank. With Glenn Hoddle, too nice for his own good. With David Gower, elegant but unsatisfying. With Colin Montgomerie, the permanent under-achiever. Fine sportsmen all, but not ones you would put your shirt on in a betting shop.

So why, emotionally, do we put our shirts on Henman? Why, as a nation, do we invest such an extraordinary amount of capital in the pipe-dream of a Henman triumph?

Why the acreage of newspaper coverage devoted to that pipedream? Why the flag-waving? Why the screaming? Why the people in sleeping-bags on the pavements outside Wimbledon, craning for a glimpse of their hero?

It is not healthy. It cannot be healthy.

For Henmania, like similar diseases of the brain, is highly contagious. It can strike anyone at any time - as I can testify from personal experience, having caught the virus myself.

Somewhere, gathering dust in my cupboard of old sporting videos, is a tape of Henman beating Todd Martin in a nail-biting match on Centre Court. As he wins the final point, I can be seen in the crowd, jumping up and down like a demented three-year-old.

What possessed me? What made that middle-aged man in a cream jacket roar his head off and punch the air as if England had just won the World Cup? It was a good match, but not that good.

I can only imagine that I went bananas because everyone else was going bananas. Laboratory mice behave in similar fashion.

And what does it say about us, as a people, that we give our hearts so profligately to a habitual loser? It is almost as though, in that epitome of English niceness, with his nice haircut and his nice wife and his nice baby, we see our own limitations mirrored.

In America or Australia, Henman would be a minor sporting celebrity, no more. He would not be lionised because he does not deserve to be lionised.

It is only in Britain that gallant losers are wreathed in such an aura of sentimentality. In our hearts, we expect to lose, so we condition ourselves for defeat.

We shy away from the animal ferocity of Pete Sampras, say, and console ourselves with the virtues that Henman so epitomises: the ability to try hard, give it our best shot, but not lose our temper if our best shot does not prove good enough. We value good manners above winning. And why not? It is a perfectly honourable attitude.

But to take that stoicism in defeat a stage further, and to idolise the plucky loser, the way Henman is idolised, is madness. We are just milking failure, consecrating under-achievement.

Today, Henman is due to play his first match - his opponent is the Czech Tomas Zib, who is ranked 157th in the world.

I wish Tim all the best. I really do. The further he progresses in the tournament, the louder I shall cheer. But is it too much to hope for a little sanity this year?

His more strident fans not only inflame expectations, piling unnecessary pressure on the player, but give the outside world the impression that Britain is a land of Union Jack-clad nutters, wallowing in masochism and self-delusion.

Wimbledon used to be a rather genteel affair, the British summer at its civilised best. One sipped Pimm's, grumbled about the cost of strawberries and tuttutted at those American loudmouths disputing line-calls.

Occasionally, a British player would take a set off the Argentinian No 3 and a little flurry of applause would make itself heard in Putney. But there was no shouting and no hysterics.

It says a lot for the excesses of the Henmaniacs that they make one feel a twinge of nostalgia for that sleepy, bland, unambitious age.

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