No. 346 · Fixing problems · Biology
Herd Immunity
Some protections only work when enough of us take part, and then they shield everyone.
In the Almanac
“The end of smallpox”, 1967–1980 · “Serum Institute of India”, 2020–2021
Both stories in full, three ways to use it and where it misleads.
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What is Herd Immunity?
Herd immunity is the protection a whole population gains when enough people are immune that a disease can no longer spread easily. It shields babies and the vulnerable, and it helped the world eradicate smallpox through mass and ring vaccination.
How do you use Herd Immunity?
The model applies beyond medicine. Some protections only work collectively, like team security habits, family rules about scams, or simply staying home when sick. Your small action becomes part of a barrier you will never see. The threshold varies and protection can fade, so it needs renewing.
When does Herd Immunity matter?
Herd Immunity is one of the 40 mental models for fixing problems, the situations where you ask: Why does this keep going wrong, and how do I fix it for good? Within that, it belongs to “How problems spread”, alongside Broken Windows Theory, Chain Reaction and Containment.
What is an example of Herd Immunity?
Two stories show it at work: The end of smallpox (1967–1980) and Serum Institute of India (2020–2021). Both are told in full in The Mental Models Daily Almanac, which has two stories for each of its 365 models.
Which mental models are related to Herd Immunity?
Free Rider Problem, Tragedy of the Commons and Critical Mass. Herd Immunity itself comes from biology.