Mind Virus

This trope is for viruses of the mind. They spread the same way as any other normal virus, either through exposure to someone who's already infected or through some airborne particulate. However, unlike normal viruses, they primarily don't cause any physical symptoms. Instead the virus will alter the way its victims think.

Mind Viruses can vary in terms of their effects and their severity. Minor cases might simply give the victims weird dreams, or give them a slight compulsion to do something (though this compulsion can be ignored with enough willpower). More severe Mind Viruses can do everything from alter or completely erase memories, alter one's personality, or even completely destroy someone's mind, leaving them a shell.

Mind Viruses usually don't have anything to do with brainwashing or mind control. A nefarious agency may have developed and released the virus, but after that the virus is uncontrolled, spreading from one person the the next in an unpredictable manner. Rather, a deliberate Mind Virus may be used as a weapon, intended strictly to disrupt. Alternatively, it might have started off with good intentions but ended up mutating and spreading out of control.

This is a trope usually found in Science Fiction, though not necessarily limited to it. In nearly all cases it's a Synthetic Plague. A specific version of this would be a Hate Plague. Not to be confused with The Virus, a trope where a contagion acts as an intelligent, self-directed entity (adding to "itself", having a Hive Mind, etc). Compare Infectious Insanity, a trope where mental illnesses is treated like a Mind Virus, and Ear Worm, where it's a song that can't get out of your head.

In Real Life, there's a sociological model known as Memetics, which posits that information and culture behaves like genetics ("meme" being the informational analogy of "gene")--that units of information transmit themselves from mind to mind. There's other competing sociological models, some of whose proponents point to Memetic Mutation as an example that Memetics isn't a viable model.

Anime and Manga

 * In Higurashi no Naku Koro ni, is caused by some sort of parasite that is initially compared to a virus in the sixth arc. That is the only arc where it is referred to as a virus, and the person who called it that was an Unreliable Narrator. The arcs after it tend to use the term parasite. Since there are many lifeforms (and viruses) that can theoretically be called parasites, it's a little uncertain what type of lifeform it actually is..

Comic Books:

 * An issue of The Authority had Apollo and Midnighter teaming up with alternate-universe versions of themselves to stop a killer meme: anyone who heard it would whisper it to the first person they saw and then commit suicide. They had to stop the meme from making its way to a TV studio, where it would be spoken on a live broadcast.
 * In one Batman Story Arc, the villains attempt to release a meme virus into the population of Gotham which will give everybody a predisposition to addiction, thus causing chaos.
 * In Planetary, the Big Bad's power is seemingly that his mind expands and 'infects' other minds near himself, causing them to start to think the same way and become, well, him. It ends up becoming an Informed Ability because we never see him use it.
 * Transmetropolitan alludes to things like this at times, although details for how they work are never given. A Canibalism Meme is mentioned as one point, for instance.

Literature:

 * Alastair Reynolds features these prominently in many of his novels. Century Rain has the "Amusica" virus that causes its victims to be unable to appreciate music (released as a demoralizing tool in a war). Chasm City has a communicable virus that inflicts its victims with the memories of a religious cult leader, giving them dreams of his life and causing stigmata to appear on their body.
 * Charles Stross has this in the novel Glasshouse. A virus called "Curious Yellow" has infected nearly all humans, via the ubiquitiuos and necessary Warp Gates that are everyone uses to get around. It deleted memories of a recent war, giving pretty much everyone amnesia of that specific time period. Furthermore, it also deleted the memories of who released it and its full extent, meaning that nobody knows what else it might've done (or still be doing).
 * Master of Space and Time by Rudy Rucker has a section where the heroes visit an alternate dimension full of "meme viruses". One of the characters is bitten by a "Jesus Lizard" and subsequently starts gaining messiah-like characteristics, along with growing out his hair and beard and wearing sandals all the time.
 * In Kathleen Ann Goonan's Queen City Jazz, part of her "Nanotech Quartet" series, a nanotech apocalypse sweeps across the world. There are several nanotech mind viruses going around, such as a virus that compels its user to go to one of several glowing spheres that have cropped up around the country. Further, the main character in Mississipi Blues is infected with a virus that compels her to abandon her old life and go rafting down the Mississippi river a la Huckleberry Finn.
 * The third book in the Plague Year Series by Jeff Carlson, Plague Zone, features a mind virus that turns its victims into mindless husks that wander around, attempting to spread the virus to others (through airborne contact). They're not violent or anything, but insistent, shambling about like living zombies that don't eat or attack.
 * Peter Watts's Rifters Trilogy features several mind viruses, most notably "GuiltTrip", which forces its victims to always do "good" things (and severely punishes them, through pain or death, for failing to do so). A government agency purposefully infects its employess with GuiltTrip, reasoning that by doing so they do not have to worry about security anymore, since nobody would dare attempt to subvert or steal anything.
 * There is a short story by Robert Sheckley where everyone on Earth learns to levitate. If, however, they ever doubt their ability to levitate, they lose it. Additionally, if one person sees another who is unable to levitate, it would automatically plant doubt into their minds as well, in effect becoming a fast-spreading virus.
 * Snow Crash features a "biolinguistic virus" that renders its victims unable to communicate normally; anytime they try to talk, they just speak gibberish.
 * Greg Bear's novel Vitals features bio-engineered viruses that manipulate their victims hormones and brain chemistry. Though the viruses themselves are undirected, the evil government conspiracy will "dose" people with different strains of the virus, in combination with setting up fake evidence, in order to discredit anyone who threatens to expose the conspiracy. One character, an investigative author who starts getting too close to the truth, is infected with a compulsive anti-semitism virus that causes him to lose his reputation. Later on, another character is made to murder by use of a Hate Plague. The main character is also infected with a kind of "love" virus that renders him dopey and lovey-dovey with respect to a woman who's an agent of the conspiracy.
 * In one Homer Price story, the town gets infected with a song. A mysterious stranger comes to town and puts a record in the donut shop's jukebox, telling Homer and his friend not to play it. Of course they do, and they can't stop singing the song. They teach the song to others, who can't stop, and so on. Homer finally cures himself and the town by learning a different song (from a Mark Twain story), which makes them forget the first song, and once you pass that song on you forget it in turn. They send the one person infected with the new song out of town (she was going on vacation anyway) where they hope she'll teach it to someone else.
 * "Hostess", a short story by Isaac Asimov, suggests that humans die of old age because they're infected by a parasite that exists in their mind (it having adapted to not even needing a body). By the end, it's unclear whether or not how much of humanity is caused by the existence of the parasite.
 * In The Curious Incident of the Dog in the Night-Time, the hero fantasizes of a virus ("like a computer virus") that spreads by people looking each other in the face. He is heavily autistic, and thus would survive such an epidemic.
 * Though the "culling song" in Chuck Palahniuk's Lullaby starts out as a lethal Brown Note, it eventually turns out that merely thinking about it can telepathically transmit it to someone else, making it a lethal Mind Virus.

Live Action TV:

 * The Star Trek: Voyager episode "Flashback" featured a virus in the form of a fake memory which spread through mental contact.

Other:

 * The Game. It even has an antivirus!

Web Comic:

 * In Schlock Mercenary, the Macarena is referred to by Reverend Theo Forbus as a "proven memetic infector."
 * A Miracle of Science gives us Science-Related Memetic Disorder, for which it was the Trope Namer. However, while the trope treats Mad Science as a medical disorder, in A Miracle of Science it's communicable.

Web Original:

 * The SCP Foundation has quite a few of these, which are classified under Memetic Hazards.
 * In Orion's Arm memetics are a mature science.

Real Life:

 * 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 labeling 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.