I looked it up.

Herd immunity is when enough people in a population are protected from a disease—either because they’ve been vaccinated or because they’ve already had the illness—that the disease can’t spread easily from person to person.

Here’s how it works step by step:

1. Diseases spread through contact Many contagious diseases (like measles or flu) move from person to person. If no one is protected, one sick person can infect many others.

2. Immunity acts as a barrier When a person is immune, the disease can’t take hold in them, so they can’t pass it along to others. Immunity comes from either:

Vaccination (safer way)

Recovering from the disease (natural infection, but riskier)

1. More immune people = less opportunity to spread If enough people are immune, the chain of transmission breaks. A sick person might come into contact only with people who are already immune, so the disease fizzles out.

2. Community protection This protects people who can’t be vaccinated or who have weaker immune systems (babies, older adults, or those with health issues). They’re less likely to encounter the disease because it simply isn’t spreading widely.

3. Threshold matters Each disease has a different threshold for herd immunity, depending on how contagious it is. For example:

Measles is extremely contagious, so about 90–95% of people need to be immune.

Polio needs around 80–85%.

Less contagious illnesses need a lower percentage.

In short: herd immunity isn’t about everyone being immune—it’s about having a high enough proportion immune that the disease struggles to survive in the community.