By underlining the potential dangers involved in creating novel pathways by which pathogens can pass from animals to humans, the author Edward Hooper has performed a useful service. In his book The River, Hooper argues that the origins of HIV lie in a contaminated polio vaccine produced from chimpanzee tissue and used in Africa in the late 1950s. Even those who dismiss this argument will accept that there has been complacency about the ease with which pathogens can cross the species barrier. The history of BSE in Britain perhaps provides the most graphic example — and as virologists weigh the potential dangers posed by xenotransplantation, Hooper has delivered a timely warning.

But in pursuing his hypothesis with such zeal, Hooper risks undermining the effectiveness of this message. This week's meeting at the Royal Society in London, at which Hooper confronted those he criticizes in his book, was at times an unedifying spectacle (see page 117). There seems no prospect of a resolution of the conflict between Hooper's insistence that he has verbal evidence that chimpanzee kidneys were sent to the Wistar Institute in Philadelphia and university research centres in Belgium, and the outright denials of the researchers involved. Similarly entrenched positions greeted the news that samples of Wistar vaccines dating from the period were produced using monkey, not chimp, tissue, and are not contaminated with the virus. The accused scientists are overoptimistic in hoping that these results will “put to rest” Hooper's accusations — the samples available today represent only some of the many batches of vaccines made. But Hooper will win few plaudits by dismissing the results as “irrelevant”.

The question of whether the AIDS epidemic was triggered by contaminated polio vaccine is a legitimate one — but for reasons of public interest, not ‘scientific’ truth. Hooper's hypothesis cannot be tested experimentally. Given this, any inquiry into the hypothesis needs to be assessed in terms of its own social costs and benefits. If the costs — for example, in undermining public confidence in vaccines, or in distracting AIDS researchers from more pressing tasks — are too high, and the benefits questionable, it may be time for a truce.