What is Herd immunity?
[[Image:Herd immunity.svg|thumb|upright=1.5|The top box shows an outbreak in a community in which a few people are infected (shown in red) and the rest are healthy but unimmunized (shown in blue); the illness spreads freely through the population. The middle box shows a population where a small number have been immunized (shown in yellow); those not immunized become infected while those immunized do not. In the bottom box, a large proportion of the population have been immunized; this prevents the illness from spreading significantly, including to unimmunized people. In the first two examples, most healthy unimmunized people become infected, whereas in the bottom example only one fourth of the healthy unimmunized people become infected.]] '''Herd immunity''' (also called '''herd effect''', '''community immunity''', '''population immunity''', or '''social immunity''') is a form of protection from [[Infection|infectious disease]]. This happens when a large percentage of a population has become [[Immunity (medical)|immune]] to an infection, whether through previous infections or [[vaccination]].<ref name=pmid21427399>{{cite journal|last1=Fine|first1=P.|last2=Eames|first2=K.|last3=Heymann|first3=D. L.|date=1 April 2011|title='Herd immunity': A rough guide|url=http://cid.oxfordjournals.org/content/52/7/911.full|journal=[[Clinical Infectious Diseases]]|volume=52|issue=7|pages=911–16|doi=10.1093/cid/cir007|pmid=21427399|doi-access=free}}</ref><ref name=gordis>{{cite book|last=Gordis|first=L.|year=2013|title=Epidemiology|url=https://books.google.com/books?id=7YX6AQAAQBAJ&pg=PA26|publisher=Elsevier Health Sciences|pages=26–27|isbn=978-1455742516|access-date=29 March 2015}}</ref> When many individuals have immunity, they do not spread the disease any more. This either stops or slows the spread of disease.<ref name=merrill>{{cite book|last=Merrill|first=R. M.|date=2013|title=Introduction to Epidemiology|url=https://books.google.com/books?id=tV5LSztaS2wC&pg=PA68|publisher=Jones & Bartlett Publishers|pages=68–71|isbn=978-144