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题目材料：
Although meningitis clone lll-1 has caused hundreds of thousands of meningitis cases, it does not appear to be uniquely virulent. Now that it is possible to perform clonal analysis of meningococcal strains, it is clear that other clones have caused similar epidemics in Africa and Asia. These findings do suggest, however, that the introduction of a potentially epidemic clone under the right circumstances can be devastating. Two explanations have been given for this process: Epidemic clones randomly expand as they progress through a population, or they survive by escaping herd immunity. As an analogy to influenza outbreaks, it has been proposed that epidemics might result from what are called antigenic shifts. Although all serogroup A meningococci share the same polysaccharide, individual clones differ in the other antigens exposed on the cell surface. Once immunity to the shared antigens wanes a new clone with sufficiently different surface antigens might escape immune surveillance and start an epidemic. Epidemiologists following disease patterns will then see an "antigenic shift" as new clones supersede older clones.
以上解析由 考满分老师提供。
