In developed countries, such as Britain, HIV has increasingly become a chronic and manageable condition. The advent of antiretroviral drugs (ARVs) has allowed thousands of HIV positive people, including Chris Smith, the former Cabinet Minister, to live for many years without developing Aids.

Access to these drugs, however, has long been poor in countries such as South Africa, where 5.5 million people live with HIV and 1,000 are killed by the virus every day. The fault lies partly with pharmaceutical companies, which long refused to make ARVs affordable. But the situation has been worsened by alternative medical approaches to this plague.

Thousands of deaths can be attributed to the denialism of President Mbeki and his health minister, who have doubted the well-attested fact that HIV causes Aids. This has cleared the way for nutritional practitioners such as Matthias Rath to promote unproven vitamin pills as therapy, in place of ARVs that are known to work.

Last year, Dr Rath and an associate were savaged in The Guardian by Ben Goldacre, a practising doctor who writes a column for the newspaper. Such claims, he said, had cost lives by turning patients against life-saving medicines. Dr Rath sued for libel, but the newspaper, to its credit, refused to back down. A week ago he dropped the case, and The Guardian was awarded more than £200,000 in costs.

This was an important victory for journalism and science. First, it has illuminated a serious medical issue. While there is little evidence that most alternative and nutritional therapies have benefits, few of these endanger patients directly. They can damage and kill, but only when they are used instead of effective treatments. This is what has happened in South Africa: HIV positive people have been persuaded to abandon drugs that work in favour of quack remedies that do not, and many have died as a result. The Rath case, however, also has more general significance, for it exposes a critical difference between science and its imitators which can be of great help in judging the validity of medical claims.

The point is well made in Goldacre's new book, Bad Science (HarperCollins, £12.99) - which Dr Rath, in a glorious irony, has now done much to promote. It is that true scientific inquiry promotes open debate and dissent, rather than stifling it.

Peer review, by which new research papers are independently refereed before they are accepted by journals, is often thought to be the main bulwark against scientific error. Yet while this is important, what matters more is publication itself.

By placing the methods and results of their studies in the public domain, scientists expose their work to the criticism of the entire research community. Others can examine their data, and repeat their experiments.

Scientists positively invite such scrutiny, so that mistakes or misinterpretations will eventually come to light: they do not want to be wrong. Contrast this with the approach taken by Dr Rath, and many others whose bold claims are poorly backed by evidence. Their response to criticism is often not to correct mistakes and rethink bad ideas, but to get litigious. Far from encouraging others to evaluate their work, they seek to silence those who do not agree with them.

Dr Rath is not the only exponent of this strategy. Goldacre recounts numerous occasions on which analyses of dubious medical claims have been met with legal threats from other nutritionists and alternative medical therapists.

This is the very opposite of what science should be about. Scientists who follow the proper methods face intense criticism all the time, yet they hardly ever respond by issuing writs. They listen, they look again, and where necessary, they revise their conclusions. The laboratory is no place for libel lawyers, and there is good reason to be sceptical of those who invite them in.

Mark Henderson is the Science Editor of The Times

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