Herd immunity is the concept that you don’t need 100 percent immunity in a population to wipe out a disease. Instead, if a certain threshold of people is immune to a certain disease, then even those who are not immune will have a much lower chance of catching the disease. This occurs because there are fewer and fewer people who can spread it in the community. For most diseases, the threshold is around 90 percent. In other words, once more than 90 percent of a population is immune, the disease becomes much rarer than you would expect based on vaccination rates alone. In some cases, like with smallpox, the disease might even be wiped out.
Herd immunity is helpful because it is impossible to achieve 100 percent immunity. Some individuals won’t become immune even after being vaccinated. And some individuals should not receive a vaccine for medical reasons. So there will always be some individuals in a population who are not immune to a given disease. However, if the community has passed the necessary threshold, the resulting herd immunity will still provide protection to non-immune individuals from a given illness.