Stewart et al. are right to remind us that the 17 years following the discovery of HIV have been a long time (Nature 407, 286; 2000).

Both of us lost grandparents and great-grandparents to tuberculosis. One might have thought that in the hundred or more years since Robert Koch discovered Mycobacterium tuberculosis (without fulfilling all his postulates), we would have done a little better than the state we are in today: some 1.7 billion infected, with an annual death rate of 1.8 million. Of course we could all agree on tuberculosis being caused by another, as yet undiscovered, microbe riding on the intimate coat-tails of M. tuberculosis. Then perhaps the lack of progress would make sense. Paradigm lost.

In an earlier life one of us was valet to the French philosopher Voltaire. I remember cleaning his room one day, coming across a letter to Jean-Jacques Rousseau. As a Huguenot, I rejoiced at the remark, “I disapprove of what you say, but I will defend to the death your right to say it”. What is not widely known is the next sentence: “My only question, Sir, is whether the columns of Nature are appropriate?”.

We are staunch believers in the right to free speech, but is Nature the appropriate place to militate in favour of the pre-Copernican model of the universe or the existence of phlogiston? After all, there is Speakers' Corner in Hyde Park, when it's not raining. To demand the right of reply or equal time on such matters is a trick the creationists have used.

HIV causes AIDS. Problems arise when the proposed alternative costs lives.