An infohazard is a discrete piece of information that, by itself, causes major, fundamental shifts in your worldview. It does not need to be true, or meaningful. Bostrom provides a more detailed taxonomy.
It’s a common theme in fiction — particularly horror fiction & the ‘weird’. For instance, the “Yellow Sign” in the play-within-the-book The King in Yellow causes those who see it to become thralls to the titular King, and the play of the same name in that book causes anyone who reads its fragments to go mad. Likewise, the infohazard is a common element in Lovecraft’s work, usually as media, although in Facts Concerning the Late Arthur Jerimyn & His Family, it is a secret about the protagonist’s ancestry. However, the infohazard is not unique to horror fiction — it also appears in theology (for instance, in the form of the magical properties of the true name of god & the ability for a mental image of the ain sof to physically blind people, in jewish theology).
Infohazards are necessarily rare: humans have extremely well-developed defenses against even small changes to how they think about the world. World-changing ideas — things that could have qualified as infohazards had understanding them truly implied accepting their implications (like natural selection, for instance) — typically do not manage to fully circulate, even among professionals, until a generation dies off and a new batch (for whom the world-shaking revelation is common sense) takes its place.
