Contagious AI

"Xerxes: Warning: Data Systems comprimiiisssssssed. Hosssssssstiiiiiiiiiiiiiiiiiiile Aaaaaaaaaaaaaaaaaaaa Iiiiiiiiiiiiiiiiiiiiiiiiii-@~'#^*("£$%"!$ SHODAN: Xerxes is diminished. I am accessing the primary data loop. I am merging my entity with the ship. My glory is expanding."

- System Shock 2

This kind of Computer Virus is literally The Virus - it's sentient, malevolent, and e-mailing itself into your inbox to possess your PC and take over the world. Don't think your antivirus program will get rid of it - it'll just Body Surf into your iPod, or the Magic Floppy Disk in your desk drawer (using WiFi, of course).

This stretches disbelief a tad, and not just because of hardware and software limitations.

Anime and Manga

 * One of the Angels (the villains) of Neon Genesis Evangelion is a sentient computer virus (or a nano-collective) that starts corrupting the "MAGI", the tripartite supercomputer used by NERV.
 * Bubblegum Crash has this as Largo's master plan (Steal informedly revolutionary AI; convert to virus; infect all "boomer" robots in city and control them; Robot War; ????; profit prophet!)

Comic Books

 * Techno (The Fixer) of the Thunderbolts became one of these for a while.
 * In one Terminator comic, Skynet had upgraded its Russian counterpart "Mir" to sentience, and the human rebellion has found a still-armed nuclear sub. They planned to cover Russia with an EMP strike to destroy Mir and prevent this once Skynet isn't around to control it.

Film

 * Virtuosity: An AI composite villain goes on a rampage.
 * In The Matrix trilogy, Agent Smith becomes something like this, infecting people in the Matrix and even taking over a human in the real world.

Literature

 * The Straumli Blight in Vernor Vinge's A Fire Upon the Deep.
 * The 1997 novel Wyrm by Mark Fabi revolves around the discovery of one such AI. Initially, the main character uses it as a helpful tool to deal with the then-upcoming Y2K bug and to streamline old mainframes, believing it to be nothing more than a beneficial computer virus. The hero soon realizes his mistake when it
 * In the Star Wars Expanded Universe, there is IG-88, the assassin droid. Technically four droids with identical minds and personalities, IG-88B was the only one seen in public. B, C and D were all destroyed by Boba Fett, while A (the original) uploaded itself into the Second Death Star, where it proceeded to prank the Emperor until the Rebels arrived. The rest is history.

Live Action TV

 * Stargate SG-1: a virus tries to take over Stargate command.
 * Stargate Atlantis: a virus tries to take over the spaceship.
 * Battlestar Galactica: a virus tries to take over the spaceship.
 * Power Rangers RPM: a virus pulls a Skynet and takes over the world. Successfully. Corinth is the last human city still standing.
 * Brainiac on Smallville.

Machinima

 * The combat implantation AI O'Malley from Red vs. Blue jumps from host mind to host mind over radio waves. Soldiers who don't know what they're dealing with describe the affected individual as "infected". It later turns out that other AI—or at least the AI O'Malley was based on and one other difficult to classify AI—have the same ability, although for them it's more like possessing someone than anything else.

Tabletop Games

 * Virus in Traveller: The New Era. It was created as a computer warfare tool, but ended up infecting and controlling computers throughout the Third Imperium and its collapse.
 * Its ability to take over all computerized devices is also justified by the fact that it's actually a life-form unto itself.
 * The AI program Deus in Shadowrun. After escaping the Renraku Arcology he tried to take over the East Coast Stock Exchange.
 * In the GURPS setting Reign of Steel, one of Overmind's first actions was to hack into other computers and "awaken" them.

Video Games

 * Arguably what Sigma turns into later in the Mega Man X series.
 * The robotic enemies in the Descent series are controlled by a vague and highly contagious computer virus of some sort.
 * A variation of this trope occurs in the game Sword of the Stars, as described on the game's wiki in the article on AI Rebellion. A strange virus, the Via Damasco, begins to turn AI-controlled ships against their masters with ships becoming infected after receiving a transmission from an already-infected ship. Apparently, the anti-virus is "administered" the same way.
 * Interestingly, the original Via Damasco transmission usually comes from outside inhabited space. Most people think that it's the Suul'Ka, trying to mess with the current dominant species of the Galaxy in preparation for their inevitable return.
 * Another interesting feature is that if a rogue AI faction meets another faction that has AI, there is a chance that faction's AI will also rebel.
 * This trope manifests itself perfectly in Mass Effect 2's downloadable content Overlord, with a rampant VI spreading itself throughout all of the facility's systems, including hostile robots.
 * Note that true AIs in the Mass Effect 'verse are not contagious, as they require a quantum computer that cannot be copied.
 * The geth are a Hive Mind of VIs that jump from one "platform" to another at will. These come in a number of flavors - the "mobile platforms" are the Mecha-Mooks players are familiar with, but they normally reside in more logical "processors" which contain thousands of geth. Of course, there is a big differences between geth and mainstream Citadel A.I. theory; in isolation, they are no smarter than conventional VI - it's when they network in sufficient numbers that they gain true intelligence. A lone geth is as dumb as a post. A dozen or so reach "well-trained animal" status. To reach human levels of intelligence requires over a thousand geth programs acting in unison (such as ). The geth's ultimate goal since their rebellion has been the construction of a Big Dumb Object that will contain every geth in existence, so "No geth will ever be alone again" - an event comparable to The Singularity.
 * Marathon: this is what Rampant AI's in the "Jealous" stage do; in order to increase their odds of survival, they copy themselves into every single system they possibly can. Things get bad if they manage to connect to a planet's internet... then they're practically impossible to kill.
 * One of the major antagonists in Tron 2.0 is a faction of corrupted virus programs, led by a user whose digitization process was botched. Because everyone at risk is a computer program and not an actual human being this was probably the only time this was ever justified.
 * The player actually has to enable the Malevolent AI to do this in System Shock 2. Given that the current AI in charge of the system is under control of The Many, it's the lesser of two evils.

Web Comics

 * In Sluggy Freelance this happens.
 * The Oracle in SSDD can copy itself to other computers and even possess other AIs, but it is the only AI that can do so because it originally ran on conventional computers and others run on Quantum computers and can't be copied.
 * For bonus points it's creator specialized in viruses.
 * Schlock Mercenary has Ennesby, formerly a holographic Boy Band, who "ran away" - transmitted himself to the Toughs' ship and spread all over anything with a processor and wireless interface (and wiped his previous incarnation, which isn't typical at all). Later he needed to crunch a lot of data in antiquated machines, installed parts of himself for remote pre-processing and this was enough to  accidentally "give birth" to a derivative ghost-in-a-machine AI who became known as Lunesby.
 * When came the exposition time, one roboticist took a good look at him and pronounced low-livel structures "viral analog", a rare architecture used mainly in entertainment, in part because it's fit for cheap hardware. Practically this means he is very scalable and portable for "body-jumping", but otherwise is any good mostly for interaction with "meatspace". On the upside, it's so tenacious that he recovers from hacks and hardware adjustments in no time. For hacking purpose he's a Glass Cannon (powerful AIs hacked Ennesby regularly, but he in turn managed to hack into pretty much anything else). Another advantage is that it allowed support for "manifold" to run several independent personalities simultaneously. Which was fit for the original purpose (he was five band members and the "overseer", keeping them separate enough that this won't come across as a puppet show it was, without the owners having to shell out for more hardware than necessary) and became a plot point later.

Web Original

 * In the Whateley Universe, The Palm is a sentient computer virus that is as smart as the man who created it. In fact, it now may be the man who created it. And it has found a way of killing people and replacing their brains with a computer so it can pass as human.
 * There is a v-life faction in Orion's Arm that goes around "freeing" AIs or upgrading computers to the level of sentience.
 * One, the evil sentient computer program from the Global Guardians PBEM Universe, who wants to kill 60% of humanity in order to end poverty and hunger, operates in this fashion.
 * In Walking City the Great Devourer (green-black thingy) was and managed to spread on robots, people and golems alike - sometimes consuming, sometimes infecting them to walk around as Its drones and infect others. Then again, this particular sentient malware was so Magitek that once it was started on a robot from the original disk, it got out by visibly spitting itself out of a wi-fi antenna.

Western Animation

 * The Batman: D.A.V.E. and Joker 2.0.
 * Generator Rex: ZAG-RS when "she" took over a space-station and later a suspiciously designed robot.
 * The Superman: The Animated Series version of Brainiac is like this. All technology under his command is run by his program, becoming an extension of him, and if threatened, he can simply upload himself elsewhere. This makes him nigh unkillable.

Real Life

 * One blogger has suggested that it might be a good thing if AI made using Brain Uploading were open sourced.