I refer to the letter Doctors regurgitating propaganda of the day . I would like to clarify my comments regarding Dr John's continuing opposition to prevailing theories of Aids. Firstly, a grammatical quibble - when I said that HIV was the virus that caused the syndrome known as Aids, not Aids itself, I meant (in response to John's first letter ) that HIV and Aids are not the same. One leads to the other, and this leads me to the main thrust of my letter and counter-argument which is the basis of scientific knowledge and fact itself, something which Kumaraguru and John (despite my greatest respect for the latter's 25 years in the field) do not seem to understand.

Science, or scientific study, does not deal in absolutes. It is the most reliable form of knowledge we have at our disposal because it is a rigorous method of enquiry involving logic and empirical data which is systematically tested, analysed and evaluated to come up with conclusions which form the basis for what we call facts. However, its findings are not 100% perfectly accurate and eternally believable for the simple fact that its conclusions are reached with many uncontrolled variables, prone to human bias at times and because it uses a small sample of experiments to generalise claims about larger entities, in this case an entire disease and its patients. Science does not exist in many forms; it is a self-contained method which is objective and universal. Scientific fact as we know it therefore must be information that is least prone to the shortcomings of the objective scientific method as possible (making it reliable enough to accept) and also what is accepted at large by the international scientific community.

Our scientific knowledge is changing with every breath we take, as more conclusions are reached and deemed factual in this manner, and it is with this skeptical, open and rational mind that we should analyse these claims made about Aids and HIV testing. Firstly, the idea that not all Aids sufferers are positive for HIV is irrelevant for two reasons - the environment which they are often in and the conclusion from such a statement. Many of these patients are in Africa, and yes, afflicted with what is called Slim disease as John points out. What must be known, however, is that Aids diagnoses in many parts of Africa, due to scarce resources, are made often hastily and without any testing for HIV and Aids, based simply on the symptoms of the patients which are deemed those of Aids. This makes any diagnosis of Aids untenable as one must have tested positive for it with HIV testing or with a blood count. Consequently, any realisation that these patients do not test positive for HIV is simply indicative of poor diagnosis, not that the causal link between HIV and Aids is unproven.

And what if it is true that this Slim disease, with its similar manifestations, is indeed deemed Aids itself? The fact is that popular science has never proven that Aids is caused only by HIV; only that HIV causes Aids. Science does not have the capacity to prove that HIV is the only cause of Aids, and for all we may know, there may be more causative agents out there manifesting in immune disorders, TB, wasting and other symptoms characteristic of Aids. But the causal link between HIV and Aids remains strong in science's eyes and will not be challenged by isolated cases like these. The literature shows without doubt that the vast majority of Aids sufferers do in fact have HIV, which validates the importance of HIV tests (backtracking to the original source of this debate).

Secondly, if we are to question this basic causal link between the two, and the idea that HIV testing is reliable, we must gather enough evidence to debunk existing evidence. Kumaraguru may have his views, but the views of Eleni Papadopulos-Eleopulos are not widely accepted. She is not a medical doctor, and her views have been played down not only by the institution she works in but in a court of law no less, when she tried to defend her theory in the Supreme Court of South Australia! Her credibility is therefore to be taken with a pinch of salt as far as scientific evidence is concerned.

Once again though, regardless of the sources of these alternative theories, let us remember that the tests are not perfect either. However, when scientists say they are very specific and sensitive, it means that repeated trials and evidence from them shows that they are reliable enough to be used in the testing of HIV, and that that reliability is beyond reasonable scientific doubt. This same reasoning is in all facts of life. Do we believe for a fact that Sept 11 was caused by the Americans, as a section of Americans share that opinion? Of course not. There is enough established evidence, despite certain shortcomings, to point us in the direction of the Al-Qaeda. Do we disbelieve Darwon's theory of evolution because there are a few well-publicised holes in it? Of course not. The theory is tight enough and good enough to explain most aspects of our evolution beyond reasonable doubt.

Hence, certain dissenting studies and surveys - regardless of whoever is conducting them and where or when - must have conclusions strong enough and be able to demonstrate over and over again in many other studies that there is enough doubt in HIV testing and the causal link between HIV and Aids is untenable. This has not been done, and until then John, Kumaraguru, South Africa's Thabo Mbeki and other dissenters must be content to acknowledge that however much they might believe in these theories, they have not been proven beyond doubt in the scientific community's eyes. There is a long line of proof dating back to the first days of Aids in official history (through the contact tracing done to identify the disease's spread and to our present understanding of Aids) to suggest the facts we believe in today. It is for this reason that HIV and HIV testing will continue to remain relevant in healthcare, until a body of evidence surfaces debunking them.