Infectious Insanity

Alice is driven insane just by talking to (or otherwise interacting with) an already insane Bob. If Alice goes insane because of a particular bit of information that drove Bob insane in the first place, it's also an example of Go Mad from the Revelation. Hannibal Lecture is a popular way of transmitting this "infection".

Critical Psychoanalysis Failure is a Sub-Trope, so examples where this happens to psychiatrists trying to help madmen only go there.

Cf. Go Among Mad People, where this is invoked on purpose... and very often becomes this trope, either because madness is really infectious or just because of the level of "care" in the particular Bedlam House.

Anime and Manga

 * When in Soul Eater, his madness infects people across Shibusen, most notably.
 * Hao from Shaman King is insane but particularly good at striking people at their emotionally weakest, having built a seemingly random band of spiritually aware random passers-by with otherwise wildly varying ideals. They're all insane in that they all want to kill every non-shaman on Earth, but behave like perfectly normal people around other shamans.

Film

 * 12 Monkeys: "Telephone call? Telephone call? That's communication with the outside world. Doctor's discretion. Nuh-uh. Look, hey - all of these nuts could just make phone calls, they could spread insanity, oozing through telephone cables, oozing into the ears of all these poor sane people, infecting them. Wackos everywhere, plague of madness."
 * In Ernest Saves Christmas, Ernest and Harmony go in disguise to try and free Santa Claus from jail. By the time they get there, Santa has already convinced the other inmates of his identity (to the point they're singing carols!), which Ernest uses as an excuse to get him out by claiming that his insanity is infectious and he belongs in an asylum.

Literature

 * "Entry Taken From A Medical Encyclopaedia" (also known as "Buscard's Murrain") by China Mieville. When a specific word is spoken with the correct pronunciation, the speaker is infected and will continue to speak the word out loud.
 * The first story in The Martian Chronicles deals with a telepathic society in which insanity is highly contagious. As a result, when you think someone is crazy (because they're claiming to be from another planet, for instance) the automatic reaction is to shoot them immediately. And possibly then shoot yourself, just to be on the safe side.


 * In the Discworld novel Hogfather, HEX contracted psychosis after talking to the Bursar, which was something of a backfire since talking to HEX was supposed to alleviate the Bursar's. Ridcully, in a flash of insight, convinces HEX that it's been given a very large dose of dried frog pills (the Bursar's normal treatment), which puts it back in working order.

Live-Action TV

 * In the 1985 The Twilight Zone episode "Need to Know", insanity is spread throughout a small town by the repeated utterance of the meaning of life.
 * In the Firefly episode "Bushwhacked" it is shown that this can happen to survivors of Reaver attacks. The horror of the experience drives them into becoming Reavers, themselves.

Radio

 * A mild case in The George Burns and Gracie Allen Show: anybody who spends more than a couple of minutes talking to Gracie starts getting confused about what they're talking about.
 * No, anyone who spends a couple of minutes talking to Gracie gets confused. Those who talk to her more than a few minutes risk going completely bonkers. A studio executive who spoke with her required "two strong men" to carry him away to the sanitarium.

Tabletop Games

 * Clan Malkavian from Vampire: The Masquerade are walking catalysts for this trope. Sure, spreading madness is their vampiric superpower - but not all of them are in control of it, or even aware of what their capabilities. Indeed, before the Great Prank was revealed in 1998, even the Elders of the Clan had no idea that they possessed the Dementation Discipline.
 * The Blood Angels from Warhammer 40,000 got a lovely case of this known as the "Black Rage." It's literally the psychic imprint of their gene-father the Primarch Sangiunus fighting almost to death in defense of the Emperor's Palace during the Horus Heresy, and then actually dying fighting Horus himself. It tends to make those that share his gene-seed randomly start living out his last moments, which turns them into terrifying close-combat assailants. It's also almost incurable, taking all but literal divine intervention, and even then it's only happened twice in 10,000 years.

Video Games

 * In the Warcraft lore and World of Warcraft, it seems like any time a dragon Aspect goes bonkers, almost everyone else in the dragonflight are afflicted with the same insanity their leader/king/parent/etc. has been. They don't even have to be near or around the cause of their leader's corruption into insanity; they just seem to be like some sort of magical or psychological sponge when it comes to their Aspect. The only non-evil black dragons around are the ones who were brought up in another dimension, which presumably removed them from Deathwing's influence.
 * Insanity spreading within a dragonflight is questionable. Dragon Aspects are the most powerful members of their dragonflight and are the political and social leaders, but the entire flight is also linked to and responsible for protecting its domain (the earth, for example, or magic), so insanity spreading from the leader and most powerful member of a group to the followers might be entirely justified.

Western Animation

 * In the My Little Pony: Friendship Is Magic episode "Party Of One," and so surrounds herself with imaginary friends, using flour bags or turnip-buckets for the bodies. When Rainbow Dash comes over, Pinkie uses her voice to supply the voices for her imaginary friends, and hides behind her "pile of rocks" friend to call Rainbow Dash a chump. Rainbow Dash responds by asking "who are you calling a chump, chump?" to the pile of rocks, as if she were catching some of Pinkie's insanity.
 * In Adventure Time With Fionna And Cake, Fionna tells Cake not to touch the Ice Queen's tiara because she "might catch her crazy", which sounds like she's talking about "power corrupting" and whatnot since the tiara (and crown, in the original's case) is the source of her magic power..

Real Life

 * Truth in Television: There are forms of certain mental illnesses (such as schizophrenia) that arise from being in contact with a sufferer of another form of the illness. The historical term is folie a deux (literally "madness for two").
 * Rabies could count as this.
 * In the Middle Ages it was believed that all forms of madness were contagious like any other disease. This was exploited by at least one town who didn't want the king to pass through (since they would be liable for a levy) and pretended they were all insane to make his courtiers think again.