Among the devastating consequences of AIDS has【21】______ its epidemic spread in the develo

Among the devastating consequences of AIDS has 【21】______ its epidemic spread in the developing world. The disease has caused 【22】______ suffering, debilitation, loss of life and disruption of family, social and economic 【23】______ Because of the considerable expense and logistical difficulty in providing antiviral drugs to populations 【24】______ with the Human Immunodeficiency Virus 【25】______ 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 【26】______ and high expectations. With the 【27】______ of the Human Immunodeficiency Virus (HIV) as the cause of AIDS, it seemed that a 【28】______ would follow closely behind. But despite a large concerted effort, the problem has proven more difficult than 【29】______ , and progress has not 【30】______ the 【31】______ hopes. Here I review the 【32】______ scientific obstacles confronting the development of an effective HIV vaccine, and I consider 【33】______ strategies to overcome these obstacles.

It is instructive to consider the circumstances that have 【34】______ to past successes in vaccine development. The smallpox vaccine is 【35】______ the most successful inventions in the history of 【36】______ Why, 200 years ago, without the benefit of modern biotechnology, did the smallpox vaccine succeed so readily while an AIDS vaccine 【37】______ 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 【38】______ discovery, Edward Jennet noticed that milk maids who had previously contracted cowpox were 【39】______ 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 【40】______ be used to protect against smallpox.

【21】

A．on

B．with

C．been

D．about