[A] The strain of HIV that was discovered

题目：

[A] The strain of HIV that was discovered in Sydney intrigues scientists because it contains striking abnormalities in a gene that is believed to stimulate viral duplication. In fact, the virus is missing so much of this particular gene-known as nef, for negative factor--that it is hard to imagine how the gene could perform any useful function. And sure enough, while the Sydney virus retains the ability to infect T cells--white blood cells that are critical to the immune system’s ability to ward off infection--it makes so few copies of itself that the most powerful molecular tools can barely detect its presence.

[B] If this speculation proves right, it will mark a milestone in the battle to contain the late-20th century’s most terrible epidemic. For in addition to explaining why this small group of people infected with HIV has not become sick, the discovery of a viral strain that works like a vaccine would have far-reaching implications. "What these results suggest," says Dr. Barney Graham of Tennessee’s Vanderbilt University, "is that HIV is vulnerable and that it is possible to stimulate effective immunity against it."

[C] But as six years stretched to 10, then to 14, the anxiety of health officials gave way to astonishment. Although two of the recipients have died from other causes, not one of the man’s contaminated blood has come down with AIDS. More telling still, the donor is also healthy. In fact his immune system remains as robust as if he had never tangled with HIV at all. What could explain such unexpected good fortune

[D] At the very least, the nef gene offers an attractive target for drug developers. If its activity can be blocked, suggests Deacon, researchers might be able to bring the progression of disease under control, even in people who have developed full-blown AIDS. The need for better AIDS-fighting drugs was underscored last week by the actions of a U. S. Food and Drug Administration advisory panel, which, recommended speedy approval of two new AIDS drugs. Although FDA commissioner David Kessler was quick to praise the new drugs, neither medication can prevent or cure AIDS once it has taken hold. What scientists really want is a vaccine that can prevent infection altogether. And that’s what makes the Sydney virus so promising--and so controversial.

[E] A team of Australian scientists has finally solved the mystery. The virus that the donor contracted and then passed on, the team reported last week in the journal Science, contains flaws in its genetic script that appear to have rendered it harmless. "Not only have the recipients and the donor not progressed to disease for 15 years," marvels molecular biologist Nicholas Deacon of Australia’s Macfarlane Burnet Centre for Medical Research, "but the prediction is that they never will." Deacon speculates that this "impotent" HIV may even be a natural inoculant that protects its carriers against more virulent strains of the virus.

[F] But few scientists are enthusiastic about testing the proposition by injecting HIV--however weakened--into millions of people who have never been infected. After all, they note, HIV is a retrovirus, a class of infectious agents known for their alarming ability to integrate their own genes into the DNA of the cells they infect. Thus once it takes effect, a retrovirus infection is permanent.

[G] About 15 years ago, a well-meaning man donated blood to the Red Cross in Sydney, Australia, not knowing he has been exposed to HIV-1, the virus that causes AIDS. Much later, public health officials learned that some of the people who got transfusions containing his blood had become infected with the same virus; presumably they were almost sure to die.

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