Herd Immunity for Smallpox

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A population has "herd immunity" when the immune population is high enough so that if an infection is introduced, it dies out without building up. You can see this by sliding the initial immune fraction until the red graph does not increase in the illustration for smallpox.


The limit represents the fraction of the population left susceptible after an epidemic. You can see this by sliding the final time until is effectively zero.


Contributed by: Keith Stroyan (March 2011)
Open content licensed under CC BY-NC-SA



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