Herd immunity is a form of indirect protection from infectious diseases that occurs when a large percentage of a population becomes immune, either through vaccination or previous infections. When enough people are immune, the spread of the disease is slowed or stopped, which helps protect those who cannot be vaccinated, such as infants or individuals with certain health conditions. Achieving herd immunity is crucial for controlling outbreaks of diseases like measles or COVID-19. Vaccination programs are often the most effective way to reach this immunity level, as they help build a community's defenses without causing illness in healthy individuals.