In the ２０years since the first cases of AIDS were detected, scientists say they have learned more about this viral disease than any other.

    Yet Peter Piot, who directs the United Nations AIDS program, and Stefano Vella of Rome, president of the International AIDS Society, and other experts say reviewing unanswered questions could prove useful as a measure of progress for AIDS and other diseases.

    Among the important broader scientific questions that remain：

    Why does AIDS predispose infected persons to certain types of cancer and infections？

    A long-standing belief is that cancer cells constantly develop and are held in check by a healthy immune system. But AIDS has challenged that belief. People with AIDS are much more prone to certain cancers like non-Hodgkins lymphomas and Kaposi’s sarcoma , but not to breast, colon and lung, the most common cancers in the United States. This pattern suggests that an impaired immune system, at least the type that occurs in AIDS, does not allow common cancers to develop.

    What route does HIV take after it enters the body to destroy the immune system？

    When HIV is transmitted sexually, the virus must cross a tissue barrier to enter the body. How that happens is still unclear. The virus might invade directly or be carried by a series of different kinds of cells.

    Eventually HIV travels through lymph vessels to lymph nodes and the rest of the lymph system. But what is not known is how the virus proceeds to destroy the body’s CD-４cells that are needed to combat invading infectious agents.

    How does HIV subvert the immune sys-tem？

    Although HIV kills the immune cells sent to kill the virus, there is widespread variation in the rate at which HIV infected people become ill with AIDS. So scientists ask：Can the elements of the immune system responsible for that variability be identified？If so, can they be used to stop progression to AIDS in infected individuals and possibly prevent infection in the first place？

    What is the most effective anti-HIV therapy？

    In theory, early treatment should offer the best chance of preserving immune function. But the new drugs do not completely eliminate HIV from the body so the medicines, which can have dangerous side effects, will have to be taken for a lifetime and perhaps changed to combat resistance. The new policy is expected to recommend that treatment be deferred until there are signs the immune system is weakening.

    Is a vaccine possible？

    There is little question that an effective vaccine is crucial to controlling the epidemic . Yet only one has reached the stage of full testing, and there is wide controversy over the degree of protection it will provide. HIV strains that are transmitted in various areas of the world differ genetically. It is not known whether a vaccine derived from one type of HIV will confer protection against other types.

    In the absence of a vaccine, how can HIV be stopped？

    Without more incisive , focused behavioral research, prevention messages alone will not stop the global epidemic
    
    科學(xué)家說, 在第一批艾滋病例被發(fā)現(xiàn)以后的２０年里, 他們對(duì)這種病毒性疾病的了解, 已超過其他任何一種病毒性疾病。

    但是, 聯(lián)合國艾滋病規(guī)劃署的負(fù)責(zé)人彼得·皮奧和國際艾滋病學(xué)會(huì)會(huì)長羅馬的斯特凡諾·韋拉及其他一些專家都認(rèn)為, 全面考察那些未決問題, 可能對(duì)艾滋病和其他疾病治療的進(jìn)展是一項(xiàng)有益的舉措。

    這些較廣泛的重要科學(xué)問題包括：

    為什么艾滋病使患者容易感染某些癌癥和傳染病？

    一種傳統(tǒng)的說法是, 癌細(xì)胞在人體內(nèi)不斷繁殖, 但受到健康免疫系統(tǒng)的控制。艾滋病的情況卻不是這樣。艾滋病人很容易患非何杰金氏淋巴瘤和卡波西氏肉瘤, 但不易患在美國最常見的乳腺癌、結(jié)腸癌和肺癌等。這說明, 受損的免疫系統(tǒng)（至少是艾滋病患者的免疫系統(tǒng)）, 可以抑制普通癌細(xì)胞的發(fā)展。

    艾滋病病毒侵入人體后通過什么途徑摧毀免疫系統(tǒng)？當(dāng)艾滋病病毒通過性接觸傳播時(shí), 病毒必須穿過組織屏障進(jìn)入人體。這個(gè)過程目前尚不清楚。病毒可能直接侵入, 或許由一系列不同種類的細(xì)胞帶入。最終, 艾滋病病毒穿過淋巴管到淋巴結(jié)和淋巴系統(tǒng)的其他部分。但病毒是如何摧毀人體內(nèi)負(fù)責(zé)打擊入侵傳染體的CD-４細(xì)胞的, 目前還無法確定。

    艾滋病病毒是怎樣破壞免疫系統(tǒng)的？

    艾滋病病毒能殺死對(duì)付自己的免疫細(xì)胞, 但艾滋病病毒攜帶者患艾滋病的快慢卻大不相同。因此科學(xué)家提出問題：免疫系統(tǒng)中那些使人患艾滋病時(shí)間出現(xiàn)差異的因素能否被識(shí)別出來？如果能, 它們能否用于制止艾滋病患者病情的惡化, 甚至在最初就可能防止人們感染艾滋病病毒？

    最有效的艾滋病療法是什么？

    從理論上來說, 盡早治療可以提供維護(hù)免疫功能的最佳時(shí)機(jī)。但是新藥物無法徹底清除體內(nèi)的艾滋病病毒, 因此患者不得不終身服用這些具有危險(xiǎn)副作用的藥物, 而且還可能在產(chǎn)生抗藥性時(shí)改服別的藥物。因此新的方法可能建議在免疫系統(tǒng)出現(xiàn)衰退跡象時(shí)才開始治療。

    可能研制出疫苗嗎？

    毫無疑問, 一種有效的疫苗是控制這種瘟疫的關(guān)鍵。但是目前只有一種疫苗進(jìn)入了全面檢驗(yàn)的階段, 而對(duì)其會(huì)有多大的保護(hù)功能, 卻有著很大的爭議。世界各地流行的艾滋病在遺傳類型上各不相同, 目前還不清楚, 從某種艾滋病病毒獲得的疫苗能否防止其他類型的艾滋病。

    如果沒有疫苗, 能否阻止艾滋病病毒的傳播？

    如果沒有深入集中的行為研究, 僅靠宣傳是無法阻止這種全球性瘟疫的