Herd immunity is a "firewall" that could slow a viral outbreak down enough to stop a large scale pandemic. Polio is a good example of herd immunity. In parts of the third world, vaccines are still not available to the majority of the population. However, even in societies where only 50% of the people have been vaccinated for polio, we don't see large outbreaks of polio anymore.
Dr. John D. Cahill, M.D. is a physician and medical educator who founded the Center for Global Collaboration & Health Initiatives. While studying abroad in medical school and after his postgraduate medical training at Brown University, Dr. Cahill spent time in Southeast Asia & Africa.