单词	herd immunity
释义	herd immunity noun : a reduction in the risk of infection with a specific communicable disease (such as measles or influenza) that occurs when a significant proportion of the population has become immune to infection (as because of previous exposure or vaccination) so that susceptible individuals are much less likely to come in contact with infected individuals … when the percentage of people vaccinated against a particular disease drops, herd immunity begins to fail. Katherine Hobson Serious streptococcal disease does seem to be on the rise, however, perhaps because invasive strains are circulating against which we have not had time to develop much "herd immunity," as epidemiologists say. Wayne Biddle Word History First Known Use 1917, in the meaning defined above Medical Definition herd immunity noun ˈhərd-  : a reduction in the probability of infection that is held to apply to susceptible members of a population in which a significant proportion of the individuals are immune because the chance of coming in contact with an infected individual is less
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