武大英语考博阅读部分.doc

Reading Comprehension Text 1 Scientists have genetically engineered tomato and tobacco plants to produce a vaccine against the virus that causes severe acute respiratory syndrome, or SARS, the disease that killed nearly 800 people in 2003. Meanwhile, another team of researchers has developed a vaccine that protects monkeys against the deadly Ebola and Marburg viruses, which have plagued central Africa. An ongoing Marburg outbreak in Angola has killed at least 357 people. One reason it is difficult to develop vaccines for diseases like SARS and Ebola is because some viruses are believed to mutate constantly. If a virus changes quickly, a vaccine might be suitable for a while but not forever. While both vaccines are far from ready for human use, the studies raise the possibility of producing economical vaccines for diseases for which there is no known cure. There was a need to prepare quickly a vaccine that is inexpensive and safe, said Hilary Koprowski, who is the director of the Center for Neurovirology at Jefferson Medical College in Philadelphia, Pennsylvania. Health officials alike have a strong need for vaccines that are both economical and easily administered. Plants have long been seen as an attractive source for vaccines, since plants can be grown inexpensively on a large scale. The easiest plant to genetically modify is tobacco. For the SARS study, Koprowski and his colleagues engineered low-nicotine plants to produce a so-called spike protein of the SARS virus. The protein is used to trigger an immune response in the human body. Once the researchers showed that tobacco could produce the SARS virus spike protein, they applied the same technique to tomatoes. Mice that were fed genetically modified tomatoes developed antibodies to the virus that causes SARS. However, the scientists did not report whether they injected the mice with the SARS virus. It does not show that the antibody offers any protection, said Michael Lai, a virologist at the University