Editing Herd immunity (section)

==Mechanism==
Individuals who are immune to a disease act as a barrier in the spread of disease, slowing or preventing the transmission of disease to others.<ref name="merrill">{{cite book |url=https://books.google.com/books?id=tV5LSztaS2wC&pg=PA68 |title=Introduction to Epidemiology |vauthors=Merrill RM |date=2013 |publisher=Jones & Bartlett Publishers |isbn=978-1449645175 |pages=68–71 |access-date=29 March 2015 |archive-url=https://web.archive.org/web/20210412052913/https://books.google.com/books?id=tV5LSztaS2wC&pg=PA68 |archive-date=12 April 2021 |url-status=live}}</ref> An individual's immunity can be acquired via a natural infection or through artificial means, such as vaccination.<ref name="merrill"/> When a critical proportion of the population becomes immune, called the ''herd immunity threshold'' (HIT) or ''herd immunity level'' (HIL), the disease may no longer persist in the population, ceasing to be [[Endemic (epidemiology)|endemic]].<ref name="ska"/><ref name="pmid24175217"/>

The theoretical basis for herd immunity generally assumes that vaccines induce solid immunity, that populations mix at random, that the pathogen does not evolve to evade the immune response, and that there is no non-human vector for the disease.<ref name="pmid21427399"/>