Christine Maggiore, who was diagnosed with HIV in 1992, waged a long, bitter campaign denouncing the prevailing scientific wisdom on the causes and treatment of AIDS. She fiercely contested the overwhelming consensus that the HIV virus causes AIDS, and that preventive approaches and antiretrovirals can help thwart the disease's spread and prolong the lives of those who suffer from it. Her campaign ended this week with her death at age 52. Her challenge, however, continues, as Maggiore's argument -- that scientific consensus, no matter how established, remains subject to objection -- runs through debates with profound public policy implications. Does smoking cause cancer? Do human activities contribute to climate change?

It is admittedly difficult to spot the moment when a scientific theory becomes an accepted fact. It took hundreds of years for the Catholic Church to acknowledge the work of Galileo, and it still flinches at Darwin. Meanwhile, the rest of the sentient universe long ago accepted that the Earth orbits the sun, and all but the most determined creationists see the undeniable evidence of evolution at work. Still, science is a discipline of questions, and rarely is a fact established so firmly that it will silence all critics. At the Creation Museum near Cincinnati, the exhibit guides visitors "to the dawn of time" -- just 6,000 years ago. That makes for some startling conclusions, not the least of which is that dinosaurs and humans were created by God on the sixth day and lived side by side. Call it the Flinstones theory.

Of course, new questions inevitably emerge from new inquiry and new data. How, then, to judge when a theory becomes fact, when it slips beyond legitimate objection? The test lies in balance: A preponderance of evidence accumulates on one side or the other. Those who contest that evidence must demonstrate the plausibility of alternatives and produce evidence to support them. If the alternatives are implausible, they melt away. Eventually, there is nothing left to uphold the view that the sun is circling the Earth or that natural selection is a secular myth.

In some instances, these debates are interesting but not terribly consequential. But sometimes they are of staggering significance. When the theory in question is about the cause of climate change or AIDS, misplaced skepticism, whether cynical or well-intentioned, can lead to grave results. For years, the South African government joined with Maggiore in denying that HIV is responsible for AIDS and resisting antiretroviral treatment. According to a new analysis ;jsessionid=Jhfp1JywCQyb2Qp0WTbm8Rkg8l5dPcvsz8z378GSCgh5q7Hx1CQn%211355756759%21181195629%218091%21-1 by a group of Harvard public health researchers, 330,000 people died as a consequence of the government's denial and 35,000 babies were born with the disease.

Determined to reject scientific wisdom, Maggiore breast-fed her daughter. Eliza Jane died in 2005, at the age of 3. The L.A. County coroner concluded that the cause of death was AIDS-related pneumonia. Maggiore refused to believe it.