Contagions always spread through only the susceptible fraction of the population. Prior to mass communications, there were too few susceptible individuals in any given population for it to spread. The herd was immune, because any infected person would die before ever meeting another susceptible person.

Enter the Internet. Now all the susceptible people can aggregate around their own forums/boards/podcasts/groups/subreddits and infect each other with their unique strains of madness.

There's no more herd immunity, because all the susceptible people can find each other instantly.