Typhoid Mary

The Typhoid Mary is a character who carries an infectious disease but doesn't suffer any visible symptoms -- in some cases she may actually be immune to it herself -- and spreads the disease to everyone she encounters.

Unlike the Plaguemaster and Poisonous Person, the Typhoid Mary isn't a villain. She doesn't intentionally try to spread her infection, and is often unaware that she is infected at all; she is at worst Obliviously Evil. However, villains may try to use the Typhoid Mary as a weapon, deliberately infecting an unsuspecting person with a contagious disease so she can unknowingly spread it to others.

In Zombie Apocalypse works, there may be some overlap with the Zombie Infectee, depending on how the infection spreads. In most cases, however, the Zombie Infectee doesn't pose a danger of infecting others until he actually dies and become a zombie himself. The Typhoid Mary, on the other hand, is highly contagious from the first moment of her infection.

The Trope Namer is the Real Life example of Mary Mallon, the original "Typhoid Mary" who spread typhoid fever to at least 53 other people while refusing to believe she carried the disease at all, because she never became sick from it herself. In biology/epidemiology this is known as an "asymptomatic carrier" (i.e. carries the disease but shows no symptoms of it).

Not to be confused with the Daredevil villain of the same name.

Anime & Manga

 * In the second episode of Memories, a man takes an experimental drug that causes anyone near him to die instantly. He never realizes he's dangerous, and seems quite upset that everybody's trying to kill him.

Comic Books

 * Italian comicbook Dylan Dog had a short story about a girl born into a werewolf family. While not a werewolf herself, she was a carrier of lycanthropy - anyone who came into contact with her internal fluids (including through sex) would be infected.
 * Averted in an issue of The New Teen Titans; a young Russian woman is sent to run errands in New York City, while unknowingly infected by her boss to spread disease in America. Unfortunately, she gets progressively sicker as she goes along, and once the Titans catch up to her, she is the only one who can't be cured.

Film

 * Jack from The Transporter 2 was turned into one by the villains as part of an assassination plot.
 * Love Interest Nyah from Mission Impossible II was going to be used as one by the villains. She knew it, however, and was ready to kill herself rather than spread the disease. Luckily, Ethan got her the cure in time.
 * In 28 Weeks Later, Alice is an asymptomatic carrier of the Rage virus, which results in it breaking quarantine.
 * This was the central plot driver in REC, where no one knew that the little girl didn't actually have tonsilitis, but something far far worse.
 * In Kids, one of the characters is a sexually promiscuous teenage boy who doesn't know he's HIV-positive.
 * Patrick Dempsey's character in Outbreak unwittingly infects a plane-load of people he's travelling with after he catches the virus from a monkey.
 * Anthony Wong's Villain Protagonist character from Hong Kong exploitation movie The Ebola Syndrome contracts the eponymous sickness, but turns out to be a one-in-a-million case whose immune system fights it off while remaining contagious, and unknowingly spreads it around in South Africa and Hong Kong. Towards the ending, he becomes aware of his condition and starts to spread it willingly, but by then he's been long confirmed to be a Complete Monster.

Literature

 * Lenie Clark of Peter Watts' Rifters Trilogy series carries βehemoth back to the surface world. However she isn't a proper example, since she spreads it deliberately.
 * The protagonist of Peeps carries a parasite that transforms anyone he gets too intimate with into a vampire. Now that he knows, he's careful not to spread it, but he had already infected a number of people before he found out he had it.
 * Geoffrey Allen from The Changeling Plague. He gets a retrovirus engineered to cure his cystic fibrosis. Since it was specifically designed for his DNA, it has only positive effects on him. But unbeknownst to him, he's also spreading to other people...
 * The novel Code Orange revolves around a teen who fears he may be a carrier of smallpox.
 * In Wild Cards, Croyd 'The Sleeper' wakes up with the ability to spread a new strain of the Wild Card virus; one where people who have aready been transformed with Wild Card can be re-infected and changed all over again. Croyd keeps moving and spreading the virus because he had entered the paranoid stages of his meth addiction, and can't trust anyone.
 * Stephen King's The Stand has several unwitting Typhoid Marys but "Joe Bob" Brentwood is the most important as he unintentionally spreads the superflu beyond any chance of containment.
 * The rabies-spreading Rant Casey, of, well...Rant. In fact, the book compares him to Typhoid Mary several times. However, he seems to actually want to spread the disease... unless he doesn't... it's a little complex when you don't know
 * In the Temeraire series, the British government exploits this by infecting a French dragon with a highly contagious plague and sending it back to its home base to spread the disease.
 * story in Haunted 2005 is about an island facility where examples of this trope from all over the country live in sterile, high-security isolation.
 * In Wyvern by A.A. Attanasio, one character finds that every village he stays at is struck by fever, although he never gets sick himself. He is finally taken in by a Buddhist monastery, and although the monks get sick, they teach him how to treat the illness. Finally, he administers the cure to himself, and from then on nobody else gets sick by having contact with him.
 * World War Z had carriers' organs being harvested and sold on the black market, spreading the infection across the world.

Live Action Television

 * The Star Trek: The Original Series episode The Way to Eden featured Dr. Sevrin, carrier of the Synthecoccus Novae virus who was more crazy in the coconut than malicious. While the episode didn't have any reported infections, having to isolate him to ensure that did cause his followers to get very angry.
 * An episode of House had an old lady volunteer at the hospital handing out teddy bears and unwittingly spreading echovirus. She barely had any symptoms, and healthy adults were unaffected, but it was lethal to newborns.
 * In Dark Angel, Original Cindy's girlfriend was turned into one and eventually died from it.
 * The X-Files episode "Jump The Shark" is about a biological terrorism plot based on sending out two Typhoid Maries. (Unusually for the trope, they are both fully aware and willing participants).
 * Given an I'm a Humanitarian twist in another episode, when a town of cannibals catches Cruetzfeld-Jakob disease by serving up an infectee at a ritual banquet.
 * In Stargate SG-1, before the Ori gain the ability to attack Earth directly, they try infecting an SG team with a virus. When the team returns to Earth, they bring the disease back with them.
 * In an episode of Scrubs, an incompetent intern spends the whole week nearly killing patients until JD has a heart to heart with him and tells him that medicine isn't for him. But on his way out, the intern shakes hands with a recently-cured (and very beloved) patient, which gives her the infection that.
 * In another episode, three people die after organ transplants because the donor unknowingly had rabies.
 * The organ donor variant has also been used on Law and Order and NCIS, usually with black market human tissues and cancer.
 * In an episode of ER, one of the doctors spread a staph infection by not washing his hands adequately between patient visits.
 * The Trope Namer appears in 1000 Ways to Die.

Theatre

 * In the play Damaged Goods by Eugene Brieux, a man diagnosed with syphilis ignores his doctor's warnings to put off his announced marriage for a few years, and is able to keep his disease a secret until his child is diagnosed with it.

Video Games

 * In Metal Gear Solid,
 * This became a plot point in Metal Gear Solid 4 Guns of the Patriots when
 * On a wall in Left 4 Dead 2, some wall graffiti purports that some people are carriers of the zombie infection. It's loosely implied that the survivors might be carriers.
 * Confirmed in the comics for "The Sacrifice": the original Left 4 Dead survivors learn that they've been spreading the infection across the country and may have inadvertently doomed everyone who helped them in the previous campaigns.
 * Another Zombie Apocalypse example is the Dead Rising series: the plague was brought to America by orphaned children who were infected by the viral agent, then placed in a government adoption program. Then, all hell broke loose, (at least) Willamette and Fortune City went to hell, and the rest is history.
 * Morrowind: An example of this can be found while pursuing the Temple quests, the player may be asked to tell a woman blessed by the gods that she is a carrier of the Body Horror disease Corprus. The issue in this case was that she didn't believe she was because her state as a blessed person meant she didn't suffer the symptoms, but those around her would still catch it.
 * During the Mogloween 2011 event in Dragon Fable, The Hero meets a little boy named Andy. Poor Andy is the only person who managed to escape from a Zardbie outbreak, but not unscathed. For some reason the bite he suffered hasn't mutated him into a Zardbie, but anyone who kisses him (happens twice because girls find him too adorable) or anyone he bites (such as a Gorillaphant) becomes a Zardbie.
 * Zero from the Mega Man X series is the original carrier of The Virus that turns Reploids into bloodthirsty Mavericks. Not only is he immune to its effects (when The Virus becomes a semi-regular enemy in X5) but, in an aversion of Gameplay and Story Segregation, it actually empowers him. Plus, according to the story bridging the X series with the Mega Man Zero series, that kind of immunity was what made scientists try to study Zero's body, and finally create a cure.
 * In Trauma Team

Western Animation

 * Parodied in the South Park episode "Pink Eye," where Kenny's case of being a zombie was misdiagnosed as conjunctivitis. He goes through the episode without anyone (except Chef, eventually) noticing that he's causing the "pink eye" epidemic.
 * It was Halloween, so they thought he was just dressed as a zombie Edward James Olmos.
 * In The Powerpuff Girls, the Amoeba Boys catch a cold and cause a city wide epidemic. Although they are villains (or at least try to be), they did not spread the cold on purpose, and so are this instead of plague masters.

Web Comics

 * Deconstructed in The Zombie Hunters. The Infected contract The Virus through exposure to any zombie bodily fluids through an orifice or wound, but only being bitten or vomited on causes imminent zombification. Otherwise Infected can live full, asymptomatic lives, as the virus will remain dormant. However, they can infect others through their own bodily fluids, and will inevitably reanimate after death. On the Island Military Base humanity's remnants inhabit, Infected are both segregated from and forbidden from romancing the uninfected. Infected are also required to wear identifying armbands and ID tags, pass through checkpoints, and obey curfews while among uninfected, and the unskilled are exploited as highly-expendable Disaster Scavengers. (The eponymous Zombie Hunters) All residents are tested regularly, and anyone who goes off-island has to pass through quarantine and decontamination.
 * In Sinfest, Squigley is accused of being this for swine flu, as he's a pig.

Real Life

 * The Trope Namer: Mary Mallon spent nearly thirty years in forcible quarantine at North Brother Island, New York. Live typhoid bacteria were found in her gall bladder after her death.
 * The vast majority of infectious illnesses have some sort of incubation period, a time between the person catching the disease, and symptoms appearing. The person usually has a low chance of infecting others during this period (though there are some exceptions), but it does allow said oblivious infected person to go off to some new location and spread the illness there (sick persons rarely travel, limiting the spread into new areas after symptoms have appeared). Longer incubation periods, sometimes measured in weeks, when combined with today's easy long distance travel can be, shall we say, problematic, spreading unwitting sleeper agents of pandemic across the globe.
 * It's worth noting that it's not as uncommon as one might think for a person to be contagious, despite having few to none of the symptoms generally associated with the disease they're carrying. Overall health isn't always an indicator of how likely someone is to become a "Typhoid Mary", either. As such, that "head cold", those "allergies" or that feeling of being "a little more tired than usual" could, in fact, become a life-threatening illness for someone you pass it to (and their overall health isn't always an indicator of how likely this is, either). This is part of the reason why health departments have been pushing people to get the flu shot, especially with H1N1.