Hank McKinnell, chief executive of Pfizer, the largest, richest, most powerful drug company in the world, with a global headquarters in New York and close links to the US government, opted to spend several days this year being heckled and harangued by the activists at the international Aids conference in Bangkok, a biannual event that has become notorious for the trashing of drug company stalls, the panning of America and the throwing of red paint. McKinnell was the only drug company boss there.

Why would a man who earns more than $20m a year and was named 11th in the Fortune rankings of the world's most powerful business leaders, do that? Well, there were contacts to make, he says, but that was not all. "I'm really here because I believe just as we ask our parents and grandparents what they did in the second world war, I think our children and grandchildren will ask us what we did at the beginning of the HIV/Aids pandemic."

A few years ago, neither drug companies nor governments showed much concern. But in 2000, when the Aids conference went to the heart of the pandemic in Durban, South Africa, everything changed.

"I think the activism at that meeting caused the world to realise that we had to do something extraordinary here and out of that meeting and in the last four years the world looks quite different - pharmaceuticals are available at very low cost and governments are putting money in. There is not enough yet being done on infrastructure but a lot has been accomplished."

Some may be surprised by McKinnell's willingness to credit the activists, who consider him part of the problem for insisting on the sanctity of drug patents, which keep prices high. But McKinnell, like a number of high-ranking politicians and businessmen, has been to Africa

"You can't visit Africa and see the epidemic without it affecting you emotionally," he says. "I've been in orphanages with 50 to 60 one- and two-year-olds, all HIV positive. I know they're not going to survive without major intervention for more than six or seven years. All these kids want is a chance to live."

McKinnell thinks Pfizer has been misunderstood. It was not involved in the notorious South African court case in which 39 drug companies sued the government to block it from importing cheap drugs from other countries. "Pfizer was not a part of that decision. We thought that was a bad idea. We did not join the lawsuit. The basis of our business is being able to sell medicines to those who can afford them and our part of the bargain is to make them available to those who cannot afford them."

But there were negotiations. "Not really. Both sides were kind of dysfunctional frankly," he says, adding an extraordinary comment from the arch-defender of drug patents: "The pharmaceutical industry was kind of standing on their patent rights and I don't know what the government was trying to do. They're only now coming around to an effective programme. I think both sides deserved each other."

He denies that Pfizer was pushed into donating its drug Diflucan, which treats fungal infections that otherwise kill people whose immune systems are weakened by the HIV virus. He reels off figures: Diflucan is now donated in 22 countries, more than 4m doses have been supplied, 18,000 healthcare professionals have been trained in the use of the drug and 900 clinics can now dispense it in sub-Saharan Africa. "That didn't exist four years ago," he says. "I know of no other organisation that's done that."

But he is still criticised. Médecins sans Frontières and Oxfam say donations are unsustainable, and depend on the goodwill of the company, that companies like them because they gen erate tax breaks but that they come with strings attached and doctors have to prescribe what they have been given rather than asking for what they need. Rock-bottom prices for poor people are a better long-term answer.

McKinnell deftly kicks the sustainability argument into the long grass. "I don't know why anybody would think an NGO dependent on public funding would be somehow more sustainable than a $50bn corporation like Pfizer that's 155 years old," he says. "We said with the Diflucan programme that the drug will be available for patients we are serving as long as they live. Will we renege on that promise and go away? Sure, we might, just like an NGO might or a government might. Governments are running these programmes too and governments change.There's no sure thing in this world. But we have been around for 155 years."

But his hard line appears to be soft ening. He has two promising Aids drugs in the pipeline. If they get to market, will he donate them to Africa? "I don't know yet," he replied. "It depends on the circumstances of each individual market and each individual product and I don't doubt some of them would be donated."

One of those drugs, Capravirine, known as a non-nucleoside reverse transcriptase inhibitor, might be useful in cases where the virus has become resistant to other drugs. The other, known only as UK-427 (it was invented in the Pfizer UK base in Sandwich, Kent), is potentially more exciting because it belongs to a new class which attempts to block the virus from entering cells.

But this sort of research, from which Africa may benefit in the long term McKinnell says, can only continue if the rich world pays enough for its drugs. He warns that Europe is playing a dangerous game. "I'm particularly concerned about the policies of some European governments. If they're not willing to support the research-based industry, if they're not willing to pay reasonable prices for new medicines, and continue paying very high prices for old medicines, they're going to destroy the research base in Europe, which is almost done already."

Pfizer has closed research centres already this year in France, Germany and Italy. "They've basically become countries that don't welcome innovation ... Increasingly what's happening is that companies are unable to launch new medicines in Europe because the government is insisting on ever-lower prices. And that's just becoming politically unsupportable in the US."

In fact, low European prices have become a point of contention with Americans who are increasingly aware that they pay the highest prices in the world to companies making vast profits. Many pensioners now travel to Canada and Mexico to get medicines cheaper. But the answer for McKinnell is not lower US prices. Healthcare costs continue to rise in Europe in spite of the clampdown on drug prices and "hundreds of thousands of high-paid jobs" are disappearing to America, he says. "I think we may see a change in European policy. We're going to have to."

McKinnell switches easily from hardball to softball, one moment attacking European governments and the next describing how Pfizer's Infectious Diseases Institute in Kampala will tackle the infrastructure problems that the drug companies always said were a bigger obstacle to treating Aids in Africa than drug prices. It will train 200 doctors, who each agree to train 10 more.

"The point is not that this is an appropriate answer to the challenge, because it's not," he says. "But think if 100 companies or 1,000 companies did the same thing, we'd start making real progress."

He'll be there for the official launch next month, personally convinced, whatever the activists shout at him, that he will be able to tell his grandchildren he did what he could to help turn around the biggest health crisis in history. Without, of course, compromising the interests of one of the most powerful companies on the planet.