Editing Epidemic (section)

=== Changes in the host population ===
The level of immunity to a disease in a population - [[herd immunity]] - is at its peak after a disease outbreak or a vaccination campaign. In the following years, immunity will decline, both within individuals and in the population as a whole as older individuals die and new individuals are born. Eventually, unless there is another vaccination campaign, an outbreak or epidemic will recur.<ref>{{Cite journal |last1=Yang |first1=Luojun |last2=Grenfell |first2=Bryan T |last3=Mina |first3=Michael J |date=February 2020 |title=Waning immunity and re-emergence of measles and mumps in the vaccine era |url=https://linkinghub.elsevier.com/retrieve/pii/S1879625720300304 |journal=Current Opinion in Virology |language=en |volume=40 |pages=48–54 |doi=10.1016/j.coviro.2020.05.009|pmid=32634672 |s2cid=220414525 }}</ref>

It's also possible for disease which is endemic in one population to become epidemic if it is introduced into a novel setting where the host population is not immune. An example of this was the introduction European diseases such as smallpox into indigenous populations during the 16th century.<ref>{{cite web |title=Stacy Goodling, "Effects of European Diseases on the Inhabitants of the New World" |url=http://www.millersville.edu/~columbus/papers/goodling.html |url-status=dead |archive-url=https://web.archive.org/web/20080510163413/http://www.millersville.edu/~columbus/papers/goodling.html |archive-date=10 May 2008}}</ref>