In an interview in New York, his aged face barely cracks a smile. He shuffles around politely but without enthusiasm to pose for snapshots with admirers. He doesn't give credit to his foundation for the millions saved and takes none for himself.
The occasion is the launch of his annual letter – a sort of global state of the union address on behalf of the billions living hard against the precipice. At once dull and breathtaking, the letter no longer restricts itself to the achievements of the Bill and Melinda Gates Foundation. Instead, Gates – who, with an endowment of R310-billion, now spends more on public health alone, at R16-billion a year, than the World Health Organisation's budget – seems to speak unself-consciously and almost unwittingly for the whole world.
"Although we won't achieve all [of the eight international millennium development goals (MDGs) established by the United Nations in 2000], we've made amazing progress," Gates writes. "The MDG target of reducing extreme poverty by half [by 2015] has been reached ahead of the deadline, as has the goal of halving the proportion of people who lack access to safe drinking water."
Ranked by Forbes as the world's fourth most powerful person, the Microsoft non-executive chairperson now measures profit in terms of deaths prevented, and his business plan includes the goal of saving the lives of eight million children by 2020, with a heavy investment in vaccines.
Gates now bankrolls so many organisations – from funding development journalism at Britain's Guardian newspaper to American schools and South Africa's Human Sciences Research Council – that the inside joke, according to leading philanthropy and social investment publication, Alliance magazine, is that "it's easier to list the organisations the foundation doesn't support".
His gospel, and his gamble, seems clear: knuckle down on scientific solutions rather than on socioeconomic strategies, and go all out to eradicate rather than to control the major diseases.
Laura Freschi, associate director of the New York University Development Research Institute, says Gates has sucked slow-moving governments around the globe into his slipstream, to the benefit of millions – but has also warned that his role seems "to a worrying degree like a dictator, albeit a benevolent one".
Gates was in Cape Town in December, and he tells the Mail & Guardian that he came away rubbing his temples over genuine paradoxes he found there. Speaking about the slow uptake of the male circumcision campaign to reduce HIV infections in South Africa, Gates says "it's actually hard to explain, because it's inexpensive, the pilots were very good and the benefits to society are very strong".
He asks: "Why is TB so intense in Cape Town, higher even than Johannesburg? It's not due to HIV co-infection, since the province has lower HIV-infection rates relative to the rest of South Africa, and the regional health system is the best in the country. So why – is it the weather?"
Shaking his head, he wonders why South Africa is among the fastest to master the scientific research his foundation commissions but among the slowest to implement the solutions they find.
"We don't work in the education system [in South Africa] but what they've done in terms of making sure teachers are of a high quality and are doing things – they are among the worst! – should be very troubling."
He also came away from South Africa rubbing his arm, after dislocating his shoulder while snowboarding down a 30m sand dune in Table View. In a rare moment in which a smile almost appears, Gates reveals that Richard Friedland, Netcare chief executive officer, personally oversaw his treatment at the healthcare company's Blaauwberg hospital, who "then called my wife to say, 'I know Bill, he's gonna [be okay]'."
He says we should celebrate the fact that 14 000 fewer children in the world die each day than in 1990, but he doesn't rejoice in the astonishing number himself.
Asked about small regions of northern Nigeria or a particular piece of Brazilian research on the cassava tuber or circumcision in Orange Farm, South Africa, Gates gives amazingly well-informed, electric replies, even as his body and facial muscles appear disconnected from the same power source. It's not weariness.
As the interview runs 20 minutes over and he's being ushered out the room, he turns back when I say, loudly, to a minder: "Darn; I forgot to ask Bill about community health workers." Gates argues for a kind of national triage system, which uses modestly paid, trained community health workers not only to assess the needs of rural patients but also to assess which of those patients could do without their visits.
"[Health authorities in the Western Cape] are worried community health workers will be made exorbitantly expensive but they've realised some patients don't even need community health workers. Many need nothing but drugs, which they can get retail.
Others need 'light' health workers, some 'heavy' workers or clinic or hospital care – that's the tiering you need [in South Africa]," he says.
Sanitation and a circumcision
According to its website, the foundation has "invested" $1.7-billion in 13 years of operation in South Africa, with an emphasis on HIV, but also in major programmes such as sanitation and a circumcision campaign.
But, according to Catherine Burns, a researcher at the Wits Institute for Social and Economic Research, the foundation's grass-roots impact has been small in South Africa, compared to that in many other African countries.
She says management problems, low morale and "an isolationist mentality" are among the reasons why Gates has found "many grand plans stymied" in South Africa.
Instead, the foundation head sees the country as a potential exporter of health solutions: "We're going to do TB drug trials and vaccine trials in South Africa and those will be wonderful tools for the world."
Despite the commitment in his letter to finally rid the earth of polio by 2020, he reveals in the interview that he is determined to witness the last case in 2015.
With India reporting no more cases, polio is staging its last stand in Pakistan, Afghanistan and, in particular, Northern Nigeria.
There were only 250 polio cases worldwide last year compared to millions of deaths each to malaria, TB and HIV. But Gates tells the M&G that he is spending more to fight polio than on any of the mega diseases.
The hopelessly unfair but unavoidable question is: how many people with malaria or HIV might he have saved had he assigned the polio resources proportionately?
Gates says he strategises based on "dollars per life you've saved", and admits: "Polio in the present year is not a bargain – 250 cases. The next six years, it's a billion [dollars] a year."
This is how he explains the wager: "If you stop spending, polio will spread back. If we dropped down to $500-million or $200-million, you'll have some hundreds of thousands of polio cases; in other words you've wasted your money."
But the resurgent fight against the major diseases suggest they are like trying to accelerate something to the speed of light: you can get very close to total eradication, but that last 1% or 0.1% to your goal appears to consume more energy than the giant strides that got you there.
Is Gates right to spend all those extra millions trying to eradicate malaria or polio, rather than controlling them?
Either way, it's the searing responsibility behind these decisions which clothes Gates in his heavier gravity – a hyper-rational man who knows exactly how to save a life for the price of a Big Mac, and who has met the children who would have died if those few dollars went unspent.
And so he grinds joylessly forward.
Gates comes across as a man determined never to have to ask the kind of question with which Oscar Schindler tortured himself in 1945: Why had he not thought to sell his car, when the money would have saved three more Jews from the gas chambers? Or, perhaps as a man who has already visited an unnecessary grave in Ethiopia, Peru or India and resolved, as a penance, to save the world.