Herd immunity is a term that refers to the situation when a large part of the population is immune to an infectious disease, either through vaccination, exposure to a disease, or a combination of the two. The concept is that if there is herd immunity to a disease, those who are susceptible but not vaccinated are protected because it is unlikely that they will be exposed to the agent causing the disease. In other words, when a large portion of the population is immune to a disease, spread of the disease among individuals is much less likely. This results in protection for the whole community.
Reaching herd immunity depends in part on the contagiousness of the disease. A highly contagious disease requires a higher number of immune individuals in a community to reach herd immunity.
Also known as community immunity.