C HENNAI, India – They’ve met the transvestites and transsexuals, sat with them on bright drapes and cushions, and heard about sex work. Now they’re on their way to call on female sex workers, who will greet them with heartbreaking personal stories and an earbreaking drum troupe. But just for the moment they are stuck in traffic. So Bill and Melinda Gates are talking about their favorite subject: the world’s biggest challenge and what can be done.

The challenge is global health inequity. Only a fraction of the billions spent on medical research targets illnesses that affect poor countries, even though these same illnesses account for 90 percent of the world’s disease burden.

Each year some 10 million children die before their fifth birthday – that’s almost as though the population of two Marylands were wiped out annually – and around three-quarters could be saved by basic medicines that already exist. Fixing the twin injustices of skewed research dollars and haphazard deployment is the mission of the foundation set up by Bill Gates and his wife, Melinda.

Until just a few years ago, history’s most generous philanthropist was John D. Rockefeller, whose dominance of the oil industry in the late 19th century made him the Bill Gates of his time. But the Gateses have already given $28.8 billion, four times as much as Rockefeller measured in inflation-adjusted dollars, and whereas Rockefeller was two months shy of his 70th birthday when he made his first blockbuster donation, the Gateses have taken to philanthropy while still comparatively young.

Yet it’s not just the giving that’s important. “We don’t sit there and say, ‘Hey, we gave away $9 billion,’ ” Bill Gates remarks in another conversation later. “We ask, ‘How are we doing against malaria?’ ”

It’s a question that engages the couple on multiple levels. Helping the world’s poor is a moral mission, certainly: Gates describes his visit last Monday to a Bangladeshi project as “a religious experience.” Battling disease is a scientific puzzle, equally, and Gates exudes a boyish delight in the new vaccine technologies that the foundation is backing.

Commentators, particularly conservative ones, love to bemoan the post-1960s relativism that is said to dominate our culture: “Truth” is relative; there is no right and wrong. But to listen to Gates is to realize how overblown this is. The people who have made the most impact on our society as inventors and creators of wealth believe passionately in truth and falsity, right answers and wrong answers, and in the Enlightenment faith that reason brings progress.

Scientists necessarily believe in truth and reason, but so, too, do business leaders. Management – some would say, the science of management – is based on smart analysis of organizational psychology and consumer data. The Gateses revere Alfred P. Sloan Jr., the legendary boss of General Motors, who coined the anti-relativist injunction to “manage with the force of facts.”

The Gateses are not the first people to fight global diseases – the Rockefeller Foundation battled malaria and tuberculosis years ago – and it’s natural to ask whether they can succeed where others have not. But for every depressing moment during visits to health projects, there are three or four uplifting ones.

At the slum in Delhi, a nurse estimates that the proportion of women who spurn medical advice has fallen from around nine in 10 to one in 10 since “link workers” were recruited from the community to change people’s attitudes. In Uttar Pradesh, the vaccine strategists marvel at how new syringes, designed to jam after one use so they don’t spread viruses from patient to patient, have cut the incidence of painful abscesses and so boosted compliance. At the community center in Chennai, Bill Gates asks whether the transsexuals have had HIV tests. An emphatic babble greets him: All the people at the center are tested every three months.

Though it never triumphed against malaria, the Rockefeller Foundation made great strides against hookworm and sponsored the creation of a vaccine for yellow fever that saved millions of lives. The Gates Foundation has already backed vaccination drives that have saved more than a million lives since 2000. With the advantages of modern science and huge financial resources, it will one day claim successes that go beyond even that.