While it is permissible to entertain doubts about the prevailing scientific opinion on the issue of HIV infection and AIDS, or to engage in enquiry aimed at challenging it, we contend it is professionally irresponsible for scientists to testify in a court of law that HIV infection does not cause AIDS. The professional role of scientist in giving evidence as a scientist in a court of law is not to state a personal opinion but to testify where the weight of scientific evidence lies. Given the consensus view among scientists that HIV infection is highly likely to progress to AIDS in the absence of counteracting causes, it is for practical purposes beyond reasonable doubt that HIV infection causes AIDS. A jury in a criminal trial is therefore obliged to rely upon the consensus view of scientists in the first instance to determine whether HIV infection is a serious disease or endangers life and, on that basis, decide to convict or acquit. That anti-viral drug regimes are available to counteract the HIV virus may be relevant to sentencing but not to the conviction itself, since the scientific consensus establishes for practical decision that HIV infection remains a serious or life endangering disease, regardless of the possibility of counteracting its effects.

In deciding whether to convict a person for infecting someone with HIV, the burden of proof rightly rests on scientific evidence to establish a strong causal connection between HIV infection and subsequent onset of AIDS. As noted, evidence for HIV infection being strictly sufficient for AIDS is not required: what needs to be established is that HIV infection of people with all but exceptional immune systems is sufficient for AIDS in the absence of counteracting causes.

We concede that a full account of how a jury should rely upon scientific evidence remains to be developed. Nevertheless, whatever doubt may be reasonable for the purposes of theoretical enquiry does not imply that it is reasonable to doubt that HIV causes AIDS for the practical purpose of deciding a verdict in a jury trial or, more generally, for determining moral or legal responsibility.

The distinction we advance between the professional responsibility of scientists in the domain of enquiry and in the domain of practical decision, which includes public policy and determining legal or moral responsibility, is a simple one. However, it is often overlooked, not least by scientists themselves. To the examples we have cited of what we shall call "hyper-scepticism" in the sphere of practical decision may be added the contemporary examples of opposition to fluoridization of water by public authorities, opposition to vaccination, and opposition to the teaching of evolution in schools as the only established scientific theory of the origin of species. In each of these cases, doubts are cited in order to establish that public authorities ought not to compel citizens to drink fluoridated water or be vaccinated against diseases such as rubella, or ought not to permit public schools to teach Darwinian evolution as the sole authoritative theory of the origin of species.