Answer to Question 1
Ans: B
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If a substantial proportion of people in a population are not susceptible to a communicable disease, the few people who are susceptible are not likely to be exposed and contract the illness. This is a process called herd immunity.
Answer to Question 2
Ans: B
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Three stages of microbial adaptation and change occur over varying periods of time. During stage I, an epidemic occurs. The microbes enter a virgin population where hosts have no prior exposure to the organism and have few defenses. This leads to further spread in the population. Ultimately, survivors are usually left with improved defenses against reinfection. During stage II, the infection becomes endemic or continuously present in a geographic area or population of people. Routine childhood diseases are an example of such infections. During stage III, symbiosis is possible. Further adaptation occurs, resulting in mutual tolerance and sometimes mutual benefit for both the microorganism and the host. This is the preferred outcome.