Detailed Instructions: In this task, you are given a question and a context passage. You have to answer the question based on the given passage.
Q: What does herd immunity require in order to function?, Context: Resistance to infection (immunity) may be acquired following a disease, by asymptomatic carriage of the pathogen, by harboring an organism with a similar structure (crossreacting), or by vaccination. Knowledge of the protective antigens and specific acquired host immune factors is more complete for primary pathogens than for opportunistic pathogens. There is also the phenomenon of herd immunity which offers a measure of protection to those otherwise vulnerable people when a large enough proportion of the population has acquired immunity from certain infections.
A:
a large enough proportion of the population has acquired immunity