Population immunity, also known as herd immunity, occurs when a significant portion of a community becomes immune to a disease, either through vaccination or previous infections. This immunity helps protect those who are not immune, such as individuals who cannot be vaccinated due to medical reasons. When enough people are immune, the spread of the disease is slowed or stopped, reducing the likelihood of outbreaks. Vaccination programs, like those for measles or COVID-19, are essential in achieving population immunity, as they help increase the number of immune individuals in the community.