HAVE A drink with the man who relaxes on his Caribbean holiday with seven volumes of health statistics, and you might guess what to expect. Bill Gates’s tipple of choice is Diet Coke, and lots of it, while the chat can at times feel like a punishing bout of intellectual wrestling. You can struggle to get off the ropes.

Yet while Mr Gates, the man cast as the data-hungry geek and the personification of ruthless capitalism, may fit some of his stereotypes, they come in an intriguing mix.

When the Gates family are out in public, small talk is left to his wife, Melinda. His topics of choice — the ones that engage him in an instant — are technological advances, upstream science, macroeconomics and population trends. Stray into personal subjects, and his discomfort is immediately apparent. He shifts around, hunches up and — adopting the comforting habit that he has had since childhood — rocks repeatedly back and forth in his seat.

Melinda always steps in for her man (the man she tries to remember to call Bill in public, but occasionally lapses into Trey, his old family nickname). Confronted with scenes of high emotion, as the couple often are on their trips around the shanty towns and acute illness clinics of the world, Melinda moves to the front — to cradle the baby, comfort the crying mother and hear the stories. Bill prefers to hang back, listening intently to the clinicians, the sociologists and the researchers.

In manner and casual dress (slacks, a polo shirt and pastel colours), he retains the understated air of an accountant. But Bill Gates remains fearsomely proud of his achievements and has no time for inefficiency. Spend a week with the Gateses and you see a man who won’t stand for incompetence or error, day or night.

Even when he and his wife disagree on something, he is unwilling to drop the matter until proved right, or given proof of the right answer. Their arguments are always settled with a bet, normally a dollar.

Such data-driven precision is the force behind Microsoft, Mr Gates’s philanthropy and even his personal life. He may be Harvard University’s most famous drop-out — leaving less than halfway through his IT degree — but you need only spend a minute with him to see that it was his excessive drive, not lack of it, that prompted the decision.

The imagination, business acumen and obsession for detail that turned Microsoft into one of the world’s most successful companies is now fully engaged in transforming global healthcare. And while Mr Gates has always been loathe to talk about loosening his grip at the company he founded, it has become increasingly apparent that taking on the world’s infectious diseases could be his greatest legacy.

Philanthropy, as redefined by Mr Gates, is not the expansive retirement hobby of the rich and restless, but about identifying the best investments and outcomes for saving lives.

Bring up this latest achievement, and he does accept the accolade, albeit bashfully; he likes recognition, but remains put out by praise of his less tangible, less calculable accomplishments. So it was that when Melinda needed to give the billionaire who has everything a 50th birthday present, she plumped for an inspired option: a collection of thoughts and appreciations from friends and colleagues, printed in the form of his own personalised edition of The Economist.

• Sam Lister, Health Correspondent of The Times, spent a week with Bill and Melinda Gates in India and Bangladesh as they researched charitable ventures.

Times Recommends

• Lies and cover-up at MG Rover exposed by report
• High oil prices push manufacturing costs up
• Primark founder steps down as chief executive