Herd immunity is when you can't get infected by herds. If you have herd immuinity, you can walk through a herd of mad cows and never have to worry about getting mad-cow disease. If you have herd immunity, you can associate with goats and never have to worry about fainting-goat disease. If you have herd immunity, you can stand in the middle of a room full of sheeple, and never will you become a Democrat.