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Anime & Manga
- When Asura is released in Soul Eater, his madness infects people across Shibusen, most notably Stein.
- 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.
- Since the "criminal coefficient" that makes you a criminal in Psycho-Pass is heavily affected by stress, latent criminals tend to increase the coefficient of victims and onlookers, especially since people have become so accustomed to uninterrupted peace. When some people fake their displayed crime coefficient to go on a crime spree, the populous goes absolute nuts in fighting them back.
- In one of the TIPS of Higurashi: When They Cry's Meakashi-hen, Kumagai discusses this trope with Ooishi regarding Shion's diary, along with Despair Event Horizon.
"It's a diary of a lunatic. Even the ones reading it were becoming insane. That last page, I'm sorry for being born, ...would make even the people reading this jump to their deaths..."
- 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.
- "Entry Taken From A Medical Encyclopaedia" (also known as "Buscard's Murrain") by China Miéville. 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'snote . 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
- 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.
- In the The Twilight Zone (1985) 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.
- 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.
- Warhammer 40,000
- The Blood Angels have a lovely case of this known as the "Black Rage." It's literally the psychic imprint of their gene-father the Primarch Sanguinius 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.
- The ancient C'tan curse that created the original Necron Flayed Onesnote is contagious, and non-infected Necrons stay well clear of Flayed Ones when they appear and sometimes even kill them to keep the curse from spreading.
- Ork special character Mad Dok Grotsnik is so bloodthirsty that any Orks around him get caught up in his madness, so not only is he and his unit Fearless, unlike other special characters he can't leave a squad he's joined until he's the last one in it.
- In Dungeons & Dragons, the degenerate Kuo-Toa suffer from this as a result of centuries of inbreeding and the cruel regime of their patron deity. A Kuo-Toa who suddenly snaps during a religious rite or the stress of everyday life can inspire homicidal outbursts in its neighbors, so a special caste called Monitors closely watches a settlement's population for any signs of madness and exiles those whose sanity begins to break. These crazed Kuo-Toa fend for themselves on the settlement's periphery and act as the first line of defense adventurers will encounter. In other cases, Kuo-Toa clerics, called Whips, will imprison insane Kuo-Toa in dungeons beneath the temples of Blibdoolpoolp, ready to unleash them upon any trespassers - and the maddened howls of these prisoners add a unique flavor to religious services.
- 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.
- The entire reason the title Big Bad of Slender exists. So long as one person knows he exists (or possibly even knows about him in fiction) he will keep showing up. And his victims who go off the deep end go to great lengths to spread knowledge of his existence to the next unlucky soul. And people, inevitably, get dragged into it by accident, too. And with the games, there's the chance that he is bleeding over into this world. Just a possibility, of course, but still... Paranoia Fuel, much?
- The Elder Scrolls series' backstory has Emperor Pelagius the Mad. Infamous for his eccentricities, Pelagius was prone to severe mood swings and outbursts of Axe-Crazy violence. He did not show signs of madness as a child, being perfectly personable. However, his madness crept in when he moved to Castle Solitude, which was still said to be infected by the madness of his aunt, the Wolf Queen Potema.
- This is one of the creepiest aspects of ''Homestuck's Trickster Mode.
- In the My Little Pony: Friendship Is Magic episode "Party Of One," Pinkie thinks her friends don't want to be her friends anymore 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. But then later the Ice King's origin is revealed and he turns out to have been a sober, buttoned-down archaeologist before he found the crown, which he put on one day while indulging in a silly moment with his girlfriend, and as a direct result of that one act slowly became the Ice King we're familiar with today. And Fionna and Cake is a Show Within a Show that Ice King wrote.
- 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").
- 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.
- There's a good possibility that many cases of these were somewhat true, insomuch as poor medieval milling techniques could allow Ergot to infest local bread stores, leaving to fits of unknowingly drug-fueled insanity among entire townships.
- Richard Dawkins is the originator of the term "meme" and the concept of memetics. As an avowed atheist, he's written a number of articles and books using his model to liken religion to a disease—even outright labelling it as a "mind virus" in The God Delusion. Calling religion a "mind virus" has subsequently become popular among "New Atheists," who tend to be fans of Dawkins.