Among the devastating consequences of AIDS has _1_ its epidemic spread in the developing world. The disease has caused _2_ suffering, debilitation, loss of life and disruption of family, social and economic _3_. Because of the considerable expense and logistical difficulty in providing antiviral drugs to populations _4_ with the human immunodeficiency virus _5_ the world, the biomedical community is looking towards vaccines to help solve this compelling problem.

　　The search for an AIDS vaccine began more than 15 years ago with great _6_ and high expectations. With the _7_ of the human immunodeficiency virus (HIV) as the cause of AIDS, it seemed that a _8_ would follow closely behind. But despite a large concerted effort, the problem has proven more difficult than _9_, and progress has not _10_ the _11_ hopes. Here I review the _12_ scientific obstacles confronting the development of an effective HIV vaccine, and I consider _13_ strategies to overcome these obstacles.

　　It is instructive to consider the circumstances that have _14_ to past successes in vaccine development. The smallpox vaccine is _15_ the most successful inventions in the history of _16_. Why, 200 years ago, without the benefit of modern biotechnology, did the smallpox vaccine succeed so readily while an AIDS vaccine _17_ elusive? The answer lies in an experiment of nature that provided, to an astute observer, a clear direction for smallpox vaccine development. In this classic story of _18_ discovery, Edward Jennet noticed that milk maids who had previously contracted cowpox were _19_ to smallpox infection. This observation was the critical event leading to the finding that the cowpox virus cross?reacted immunologically with the smallpox virus and could _20_ be used to protect against smallpox.

　　1. A. on B. with C. been D. about

　　2. A. unpredicted B. uncontrollable C. unimaginable D. unprecedented

　　3. A. stability B. instability C. permanency D. soundness

　　4. A. harmed B. infected C. infectious D. infectable

　　5. A. everywhere B. throughout C. devastating D. occupying

　　6. A. difficulty B. concern C. optimism D. pessimism

　　7. A. confirmation B. identification C. information D. precaution

　　8. A. cure B. capsule C. medication D. vaccine

　　9. A. ever B. anticipated C. possible D. necessary

　　10. A. surpassed B. out?balanced C. matched D. rivaled

　　11. A. origin B. initial C. great D. modest

　　12. A. majority B. primary C. principal D. premier

　　13. A. potential B. initiative C. practical D. existing

　　14. A. lead B. caused C. contributed D. cooperated

　　15. A. by far B. till now C. among D. considered

　　16. A. vaccine B. medicine C. smallpox D. AIDS

　　17. A. slowly B. fails C. remains D. counts

　　18. A. accidental B. importance C. scientific D. vaccine

　　19. A. vulnerable B. resistant C. opposing D. defendant

　　20. A. safely B. therefore C. as well D. possibly

　　參考答案：

　　1--5CDBBB 6--10CBDBC

　　11--15BCACC 16--20BCCBB