With $80 billion, Bill Gates is the world’s richest man. So how does he spend his money? What makes his life worthwhile? And is his a happy marriage? Mary Riddell spends three months with the Microsoft billionaire to find the answers.

Microbes fascinate Bill Gates. On his arrival for a recent meeting with Justin Trudeau, the Canadian Prime Minister, Gates is spotted clutching a book on bacteria, presumably so that he might devour a chapter or two should any lull occur in the conversation.

When we meet shortly afterwards, Gates – though minus scientific tome – has the faintly restless manner of someone who finds micro- organisms more absorbing than some dignitaries and most journalists. Gates and I have met several times, and I think I know his approach. Though unfailingly modest and courteous, he loathes wasting a single second.

When it comes to changing the world, however, no topic is too small to engage his attention, and none too vast. He is, for instance, so intrigued by chickens and their potential to feed the hungry that he recently addressed a conference accompanied by a coop full of the birds.