This trope is for when a disease is used as a deterrent, preying on the fears of those being tricked in order to escape capture. The idea behind it is taking advantage of other people's survival instincts.
You've finally infiltrated a city or stronghold. You're looking around wondering what to do next when suddenly you hear a voice telling you to stop. It appears a guard or other equivalent Mook has just happened to pass by and catch you. You're suspicious as it is, but if he looks more closely, he'll realize you're an enemy of the state or you have some incriminating evidence on you that you don't want him to find. What do you do?
Fake a disease, of course. It doesn't even have to be a real disease, as long as it sounds appropriately scary. Gorapoxacephalitis will work just fine, then start shambling around, act disconnected from the world, and mention it's contagious, the guard will go running in the other direction leaving you alone. In fact, you get bonus points if you made up the disease on the spot and it came entirely from your imagination. Extra bonus points if the guard mentions that he's heard of the disease before.
There's also a version where a character is in danger of being raped, and she deceives the rapist into thinking she has an STD, so that, if the ruse is successful, the rapist loses interest. In a less dramatic version, A woman could turn down a man's pick-up lines by claiming to have the disease.
It's possible that this trope is more common in parody, where the disease used is so obviously not a real disease, it's funny that the guard falls for it.
Compare Playing Sick, Hypochondria.
Tintin has claimed that Snowy had rabies in order to scare people away.
A rare villain example was featured in Prisoners of the Sun where the crew of the ship where Calculus is being held put up a quarantine flag and have a crooked doctor declare the ship out of bounds. However, Tintin isn't fooled.
Batman #163 has Robin fake smallpox to scare some Mooks away and escape.
Played with in Garulfo: in order to get by unquestioned, the heroes (one of which is in the body of a frog) wrap themselves in a moldy shroud, shaking a clacker and moaning "Leper! Leper!". Then they meet a Genre Savvy guard, who knows better than to back away, and knocks off the hood... to see a pair of bulbous yellow eyes on a wizened green body. He runs, fast.
In the Danny Kaye vehicle The Court Jester, the lady Jean (played by Glynis Johns) tells the lecherous king (played by Cecil Parker) that she is unused to male attention because of the fear of "Breckenridge's Scourge." Breckenridge being her poor departed father, but don't worry, sire, they say it's not contagious. Oh, the unspeakable agony...
The Cure has a rare case where the person pulling off the trick actually is ill: Joseph Mazzello's character, who is HIV positive from a blood transfusion, chases off an attacker by cutting himself and chasing the bad guy away while screaming "My blood is poison!"
In Schindler's List, Stern adopted the habit of constantly scratching his head to give the guards the impression that he has lice. In several scenes, guards are seen taking steps off the path when he walks by to make sure they do not get close enough to catch it.
In Fight Club, Tyler doesn't claim he has a disease, however in modern times you don't want to be sprayed with a stranger's blood:
Tyler Durden: [his face is soaked in blood; he is shaking it over Lou and screaming] You don't know where I've been. You don't know where I've been.
In Sondheim's A Funny Thing Happened on the Way to the Forum, this device is used twice; once to get the female love interest away from her owner by claiming that she has a terrible, deadly disease to which the protagonist, fortunately enough, just happens to be immune, and once when said owner, trying to avoid the irascible customer who has paid in advance for the girl, pretends to be a leper to keep everyone away. Sight gags involving fake limbs being thrown at people abound.
In Tommy Boy, the main characters escape ticketing and/or arrest by a similar ploy (claiming they are being swarmed by bees).
In Alien³ Clements plays with the fear of an outbreak of cholera in the prison complex in order to convince his superior to allow him to perform an autopsy. Problem is, cholera has been extinct for centuries and said superior knows that.
Swiss Family Robinson: while salvaging the shipwreck, the father chases off attacking pirates with a quarantine flag, explaining to his sons the flag means Black Death is aboard.
In one of the The Thin Man movies, Nora is surrounded by admirers and Nick wants to have a word in private with her.
Nick: Now Mommy, you know you shouldn't be out of bed so soon. What would the doctors say?
Nora: (catching on) I won't go back into quarantine, I don't care who catches it!
Everyone around them makes excuses and leaves.
In Ferris Buellers Day Off, Ferris' sister uses the the house intercom to tell the "intruder" downstairs that she has her father's gun and "a scorching case of herpes."
Older Than Radio: In The Adventures of Huckleberry Finn, a pair of slave catchers are going to come out to the raft to take a look-see. Huck begs them to and implies that his father is aboard, deathly ill with smallpox. Not only do the slave catchers change their minds about checking out the raft, they give Huck $40 to assuage their consciences.
Richard Adams' Watership Down has one story of El-ahrairah wherein he uses the disease "Lousepedoodle" to steal a king's lettuce, and another in which he lies about the actual disease myxomatosis as part of a scheme to steal from a farmer.
In Martin the Warrior. Martin, Felldoh, and Brome pretend to have a fever when in the prison pit, to prevent the guards from climbing down and checking on them. Keyla the otter sings to attract the attention of Rose and Grumm, who are outside the fortress, and when questioned by a guard claims to have been singing a charm against fictitious diseases such as the dreaded "flurgy twinj". Brome even manages to mix rhymed directions for digging them out in his faked screams of pain.
In Voyage of Slaves, to keep a ship from being boarded by a naval officer who would recognize the captain as a pirate.
In Lois Lowry's Number the Stars, a casket is used as a hiding place for supplies for Jews on the run. A German soldier comes in and is told that they're holding a funeral for "Great-Aunt Birte". When he suggests that they open the casket, the main character's mother says that Great-Aunt Birte died of typhus and the doctor warned them against exposure.
In Patrick O'Brian's book Master and Commander, Dr. Maturin stops the Spanish from boarding them by expressing relief that they've shown up, now do they have a doctor aboard who understands the plague...?
In an earlier variation, Ginny patrols the corridor outside Umbridge's office in Order of the Phoenix, warning passersby about "Garrotting Gas." Too bad it doesn't work.
In the third Slayers novel, Lina and Gourry are captured by bounty hunters. When the guards start getting lecherous, Gourry very sternly warns them not to lay a finger on Lina, or they'll catch what she's got. The guards immediately back off, but Lina is not happy.
Played with in Wraith Squadron. Our heroes have taken over an enemy ship and have been posing as its crew, but they are scheduled to get a face-to-face meeting with another ship that would be able to identify the fraud. When thinking up ways to avoid the meeting the disease ploy comes up, but they think that it will look suspicious. So, they decide to turn the trick on its ear: They get the other ship to cancel the meeting due to illness, deliberately exposing them to Bunkurd Sewer Disorder. This works - the Wraiths are good at Crazy Awesome.
Another example occurs in X-Wing: Iron Fist, where Face and Phanan are masquerading as guard stormtroopers on an infiltration mission. They are stopped by the next shift of guards and are queried for a password that they don't have. Face recalls his training, which says to "provide a reason and grab as much sympathy as you can". So he immediately starts coughing and hacking away, while remaining in respectful salute to the commanding officer. It works for a moment, but they still don't know the password.
They Melted His Brain features two of the main characters bluffing their way past guards by claiming "Nailshott's Syndrome", named after one of their teachers. The two goons actually check the staff medical encyclopaedia, which reveals that Nailshott's Syndrome is a made-up disease only developed by people trying to use this trope. It then takes them several minutes to realise they've been tricked.
In Amy Tan's The Bonesetter's Daughter, LuLing fakes having tuberculosis to get past Japanese soldiers in China during World War II.
In the sequel to The Lies of Locke Lamora, Red Seas Under Red Skies, Locke and Jean pretend to be what basically amounts to lepers in order to leave a city undetected.
Also pulled in the first book, with a "plague ship" full of pirates/thieves/soldiers.
Dave Barry suggested faking a contagious and lethal illness as a way to deter insurance salesmen.
Put a sign outside your house that says CAUTION: RADIOACTIVE RABID LEPROSY VICTIM WITH SMALLPOX. This won't stop a really successful salesman from entering, but it will slow him down.
In Frederick Forsyth's The Fist Of God, a British soldier is undercover as a Kuwaiti hospital employee in downtown Kuwait during its occupation by Iraq in the Gulf War. He is stopped at a checkpoint, and gets away by whining that all of the samples in his trunk will escape into the air. He states that the samples are of cholera and smallpox.
The protagonists of Bridge of Birds use the "Plague of the Ten Thousand Pestilential Putrescences" to break out of jail.
In the Michael Crichton novel The Great Train Robbery (loosely based on a true story), conspirator Miss Miriam is disguised as a prostitute, in order to blend in with a crowd. She soon finds a bystander is interested in availing himself of her services. Rather than make a scene and break character, she returns his advances...while scratching furiously. He accuses her of having smallpox, and swiftly loses interest.
In Commodore Hornblower, a Brit working as a French privateer poses as the captain of the ship he's just taken and claims that they have smallpox aboard. It almost works, but Hornblower decides to call his bluff by sending over a surgeon, as the noise from below is too loud and insistent for weakened, feverish wretches.
In one Babylon 5 episode, Lennier claims to have "Netter's Syndrome" (a Shout-Out to executive producer Doug Netter) to drive away an obnoxious pest.
Taken a step further in Farscape. The team needed a base they'd infiltrated quarantined. So Rygel pretended to have "Hynerian dermaphollica". But to really sell it, of course, they actually infected him.
Technically they just reawakened the disease, as it was already in his system from a previous infection. And it wasn't part of the original plan, but Noranti had no choice since they were going to lift the quarantine unless Rygel showed more symptoms. The rest of the crew was understandably freaked out, since the Dermaphollica was lethal to all but one of them.
In the Mash episode in which Trapper John left, Hawkeye and Radar used this to avoid a checkpoint.
In Robin Hood, Allan a'Dale pretended to have "Turk flu" to scare off mine guards.
Star Trek: Enterprise. In "Judgement" Dr Phlox is able to get some private conversation time with Captain Archer by telling his Klingon guard that Archer has got a contagious disease. As dying from disease would be spectacularly dishonourable, the guard quickly makes himself scarce.
In Sharpe, it's used in the process of infiltrating an enemy fort. The youngest member of the Chosen Men is put on a stretcher and carried in groaning, with the resident French-speaker shouting about cholera...
Avatar: The Last Airbender had pentapox, which was actually sucker marks left by a small sewer-dwelling cephalopod called a purple pentapus. It not only scared away a single guard, but allowed for a mass evacuation of an enemy-occupied city later in the episode.
Futurama had "talking hump syndrome" when a person disguised as a hump started talking. It worked anyway ("Ah, THS.").
In one episode of Gummi Bears, Cubbi and Gusto are captured by Toadwart and the Ogres. They get the idea to paint spots all over themselves and tell the Ogres they have "Gummioleosis, and it's highly contagious". After they convince the Ogres that they've caught it as well, they give the Ogres a "cure" which involves taking three baths a day (anathema to the Ogres in and of itself) and that they must scrub the spots with "one small Ogre"...of course, the diminutive Toadwart realizes he fits the bill just as he reads these last words, and the Ogres run off chasing him.
In the classic Looney Tunes short "Hare Tonic", Bugs Bunny manages to convince Elmer Fudd that he has the highly contagious disease "rabbititis". Different from some of the other examples as Bugs had already escaped from Fudd, but pulled the rabbititis prank just to mess with Elmer's head.
Subverted in Penny Arcade - Gabe's ploy of telling his wife he has "Fisherman's Mouth" fails, since she's already had it once, and is now immune (or rather, Tycho tipped her off that it was a fake disease).
In True Magic, the villagers spot themselves with jam and put up a sign saying, "Visitors Beware: Peasant Pox, Highly Contagious" to keep the nobles away after the heroes leave.
In Knights Of Buena Vista, a lady asks Adriana, whose Player Character had just been crowned queen, to kiss her baby. Adriana says she think she's coming down with something, as she's trying to get through the crowd before her dangerous secret is found out.
There is some Truth in Television here. Insanity was believed to be contagious during the Middle Ages. This belief was exploited by the population of an English town who, not desiring to have the King's procession pass through (as that would result in inevitable expenses and taxation), feigned madness en masse. Seeing an entire town full of crazy people succeeded in causing the King to keep away.
During the Holocaust, Dr. Eugene Lazowski managed to save 8,000 Jews and other people by injecting killed bacteria that tested positive for Typhus into them. Using the bacteria, he caused a fake Typhus epidemic. The Nazis left the "infected" people alone because they were afraid of catching the disease. Read about it on The Other Wikihere.