Although meningitis clone III-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.

1. Select the sentence that best expresses the author's conclusion regarding the limitations of the usefulness of identifying a similar antigen through clonal analysis.





2. Consider each of the choices separately and select all that apply.

The passage supplies information for answering which of the following questions?

A. How can we be certain that different epidemics originated in the same cell?
B. Is it possible to develop a method of identifying all clones? ✓
C. Is the mutation of meningitis strains a previously unstudied phenomenon?



3. According to the passage, an antigenic shift takes place when

A. an epidemic causes specific clones to alter their surface antigens so that they are undetectable
B. shared antigens begin to be outnumbered by different antigens, thus allowing certain clones to pass through a population
C. certain clones are able to sidestep a weakened herd immunity and advance through a population
D. clones no longer randomly progress through a population but rather direct themselves toward the weakest elements
E. certain clones build resistance to herd immunity and share this ability with other clones through their antigens