# Herd immunity

When enough people are vaccinated that a disease cannot spread easily, which protects even those who are not vaccinated.

Biology (school) · Part 04: Health and disease · Card 7 of 10 · Last updated 2026-10-19

## Analogy

A forest where firebreaks stop the fire reaching every tree.

## In practice

Babies too young for a vaccine are protected if most people around them are vaccinated.

## Practice

### Flashcards

- **What is herd immunity?** When most of a population is immune, so the disease struggles to spread.

## Questions people ask

### What percentage of people need to be vaccinated for herd immunity?

It depends on how easily the disease spreads. Highly infectious diseases such as measles need around 95 percent of people to be immune. Less infectious diseases need a lower share. If vaccination rates fall below the needed level, outbreaks can start again.

### Who is protected by herd immunity?

Mostly people who cannot be vaccinated, such as newborn babies and people with weakened immune systems. If most others around them are immune, the pathogen has few people to infect and cannot easily reach them. It protects the community, not only the vaccinated individuals.

## Related concepts

- [Immune system](https://studyvisually.com/learn/science/biology-school/biology-school-part-4/biology-school-part-4-concept-4)
- [Antibody](https://studyvisually.com/learn/science/biology-school/biology-school-part-4/biology-school-part-4-concept-5)
- [Vaccination](https://studyvisually.com/learn/science/biology-school/biology-school-part-4/biology-school-part-4-concept-6)

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