Final answer:
Herd immunity is the protection against infection that nonvaccinated individuals receive when a large portion of the population is vaccinated, reducing the spread of the disease and protecting those who can't be vaccinated. It relies on high vaccination rates to ensure few people are susceptible to the disease.
Step-by-step explanation:
Herd immunity refers to the protection from infection that is extended to nonvaccinated individuals because a high percentage of people around them are vaccinated and thereby immune to the infection. This phenomenon occurs when the majority of a population is immune to a disease, either through vaccination or previous infection, providing indirect protection to those who are not immune.
When a significant portion of a population is immunized, the spread of the pathogen is contained, making transmission to susceptible individuals less likely. It's critical to maintain high vaccination levels in a population to sustain herd immunity and protect those who cannot be vaccinated, such as individuals with compromised immune systems.
High levels of vaccination lead to fewer susceptible individuals within the community, significantly lowering the chance of an outbreak. However, this protection can be compromised if vaccination rates drop, as observed in recent outbreaks of whooping cough and measles, underscoring the importance of vaccination programs in maintaining herd immunity and public health.