The history of this belief, called AIDS heresy, is complex, but for libertarians the touchstone issue is not the science of epidemiology but the government research machine. In reality the research in question has been done and every reasonable hypothesis regarding AIDS causation has been eliminated, except the viral theory. In 1984 non-viral AIDS causation was a justified speculation, but only a dreamer can hold this position now. The Food and Drug Administration is hated second only to the FBI and its dealings in Waco among Austin libertarian dreamers. The dreamer faction basically believes that the FDA is controlled by pharmaceutical companies and only permits dangerous and expensive drugs to make efficacy claims and go to market.
Reality is different.
If the FDA were to privatize tomorrow and all their findings were to be made advisory instead of legally binding, they would not change any procedure they use to evaluate drugs. No one would want them to. Medicine is complex, and thus expensive, by nature. AIDS is not going to going to evacuate the body by simple behavior modification. Parasitic flukes do not cause cancer.