system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is herd immunity and how does it work?
gpt
Viruses and bacteria transmit from person to person.

With a large population there will always be some people who cannot get vaccinated for a variety of reasons (such as being immunocompromised following radiation therapy, organ transplant, is allergic to the vaccine, too young for a vaccine, etc)

Herd immunity happens when the majority of the population has been vaccinated or otherwise immune to contagious disease. Lots of vaccinated people means the disease is not constantly hopping around from person to person.  When a person who is not vaccinated is surrounded by people who *are vaccinated*, this reduces their possible exposure to sick people.

It does not stop a sick person from *directly* exposing a disease to a vulnerable person, but *if almost everyone is vaccinated* then it greatly reduces the numbers of sick people and carriers.