Similar principle from what it sounds like. The idea of something that exists in such overwhelming numbers that even if each individual isn't too much of a threat the whole of them are essentially unstoppable. In most of the stories people write back home they can infect people and take them over, but not always. Sometimes they just kill. There's probably social scientists who could write whole chapters on the psychology of why the concept pops up so much but all I know is it's a really popular theme in apocalyptic fiction.