But documents that came to light during tobacco litigation in the United States suggested that this calculated and well-financed strategy went beyond ordinary lobbying. A WHO report said tobacco companies "sought to divert attention from the public health issues raised by tobacco use, to reduce budgets for the scientific and policy activities carried out by WHO, to pit other UN agencies against WHO, to convince developing countries that WHO's tobacco control programme was a 'first world' agenda carried out at the expense of the developing world, to distort the results of important scientific studies on tobacco and to discredit WHO as an institution."

Tobacco companies rose from being the object of suspicion to public enemy number one in the eyes of Gro Harlem Brundtland, WHO's director general, and her staff. The infiltration should have been a salutary experience for a benign organisation, but there are fears that the lessons may not all have been learnt. Tobacco may have been locked out but pharmaceutical companies now sit at the top table. There is a world of difference between the two industries, it will rightly be argued - tobacco is a killer, medicines save lives. But they have something fundamental in common: their duty to their shareholders, to make money.

Brundtland and her closest advisers deserve admiration for their determined efforts to spend a lot more money on health in the developing world. Brundtland's macro-economic commission on health, chaired by Harvard economist Jeffrey Sachs, last month said a further $8bn a year was needed.

It's an enormous amount of money to raise from reluctant donor governments. Brundtland is convinced of the need to have the private sector on board. Microsoft's Bill Gates, with his donations of billions of dollars to vaccine development, has done more than anybody to make this policy look a winner. But the pharmaceutical companies that have agreed to donate some medicines to poor countries and cut the costs of others are not acting purely out of selflessness.

What's in it for them? Most obviously, there is the global fund for Aids, TB and malaria set up by UN secretary general Kofi Annan, which has a purse now worth close to $1bn which will be spent on medicines and preventive measures such as condoms and mosquito nets. The model for the fund is the Global Alliance for Vaccines and Immunisation (Gavi). Save the Children Fund UK recently publicly warned of conflicts of interest within Gavi, which has vaccine manufacturers sitting on the board.

The pharmaceutical industry has lobbied hard for seats at the global fund table too. Brundtland is very much in favour of public private initiatives such as Gavi, which forge ahead (thanks to Gates's money) in a way UN agencies cannot and get things done.

That could lead to tears, say some WHO staff and outside observers. They fear this new reliance on corporate drive and cash is already in danger of skewing health policy towards vested interests.

It is most noticeable in the continuing struggle over the absence of drugs to treat people dying of Aids in Africa. Activists and organisations such as Médecins sans Frontières would like to see WHO back the use of generic drugs - cheap copies of the expensive patented medicines produced by big-name pharmaceutical companies. WHO has preferred to negotiate price-cuts with a handful of the drug giants, which are still higher than generic prices.

The macro-economic commission was also strongly against generics. One of the papers commissioned for its discussions by WHO argued that prices were not the block on the wider use of essential drugs in developing countries, blaming corruption and lack of healthcare infrastructures instead. The paper was written by Harvey Bale, head of the International Federation of Pharmaceutical Manufacturers Associations, with help from Adrian Otten of the World Trade Organisation.

The potential for conflicts of interests has been noted within WHO. The executive board has discussed guidelines for staff. But rules on free lunches are irrelevant when would-be persuaders are regulars in the staff canteen. Drug companies, baby-milk manufacturers or the big food corporations - they would all like a bit of influence at WHO. Brundtland may be right to harness their money and energy in the cause of health, but she will have to persuade her critics that it is she who is sitting in the driving seat.

· Sarah Boseley is the Guardian's health editor.