A rumor only needs one person to start it and ten people too lazy to check it.

A rumor survives on borrowed belief. One person invents it, and everyone after that repeats it because checking would take effort and repeating takes none. Each retelling makes it sound more established, since now several people are saying it. The person who started it is rarely the reason it spread.

Next time something juicy reaches you about someone at school or work, ask the simplest question available: how do you know that. If the answer is that someone else said so, the chain ends with you. That is a small thing and it is the whole defense.