This is Alice. One day, Alice gets sick.
This is Charlotte. She is healthy.
Although Alice is contagious, she cannot infect Charlotte because they do not spend any time together.
Enter Bob, a mutual friend. Alice makes Bob sick, and Bob makes Charlotte sick. Charlotte gets sick even though she never spent any time with Alice.
Let’s back up and try again. Alice is sick and Charlotte is healthy, but this time, Bob is immune. An immune Bob acts as a wall between Alice and Charlotte, and Charlotte never gets sick. The disease is unable to spread.
This is the essence of what disease experts call “herd immunity.” Although Charlotte was susceptible to the disease, she was protected by someone else in her community — her herd — who was immune.
Read the full article and watch the simulation here.