Ans: Herd immunity or population immunity is indirect protection from infectious disease. This usually happens due to either vaccination against the disease or the development of immunity in the population due to the previous infection. This means that even if people are not vaccinated or if the vaccine did not trigger immunity in some people, they could still be protected from the infection as the immune people around them act as buffers between the infection and susceptible people. Once herd immunity is established for some time and the disease’s ability to spread is hampered, it is possible to eventually eliminate the disease. This is one of the ways smallpox ott eradicated from the world.