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Typhoid Mary

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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, especially when she is an asymptomatic carrier like the original Typhoid Mary. She is at worst Obliviously Evil, stubbornly refusing to believe she is a carrier, let alone cooperate with public health authorities.

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. If she was in fact the first person who was infected, she may be the Patient Zero and possibly be the key to curing it. Thus, she can be the target of an all-out manhunt to contain her before everyone around her is infected.

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 becomes 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. For the metaphorical case of a character who unintentionally brings death and disaster to everyone they come into contact with, see Doom Magnet.


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    Anime & Manga 
  • During the Ryugu Shelter arc of 7 Seeds, it is said that the hidden carrier of the Arcia X parasitoids is likely asymptomatic and doesn't even know that they have them and are spreading them around. It turns out that the carrier is Maria.
  • Ayakashi Triangle: When Matsuri intercepts a curse meant to destroy the ayakashi medium Suzu, it instead splits Matsuri into a pair of Opposite Sex Clones who are otherwise unharmed. Later, it's discovered the curse is lying dormant in the male Matsuri, waiting to attack Suzu or any other part of the ayakashi medium.

    Comic Books 
  • Batman (Grant Morrison): In the Bad Future shown in Batman #666, Damian Wayne faces his greatest challenge as Batman when a Joker virus drives the people of Gotham insane — all except for one baby boy. Barbara Gordon tests the baby and can't find any symptoms of Joker virus at all. Damian visits a criminal named Jackanapes and demands that he help synthesize a cure for the Joker virus using the baby. Jackanapes reveals that the baby is actually a carrier of the Joker virus. Cue Joker Barbara killing the baby and opening the doors for the mob of lunatics.
  • One short story from Dylan Dog is about a girl born into a werewolf family. While not a werewolf herself, she's a carrier of lycanthropy — anyone who comes into contact with her internal fluids (including through sex) is infected.
  • Subverted 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.

    Films — Animated 
  • 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 doesn't realize that people are dying instead of just randomly going unconscious) and seems quite upset that everybody's trying to kill him.

    Films — Live-Action 
  • In 28 Weeks Later, Alice is an asymptomatic carrier of the Rage virus, which results in it breaking quarantine. Her son and daughter inherited this immunity from her, which allows the virus to spread to the continent.
  • In Borat Subsequent Moviefilm, Borat was unknowingly infected by COVID-19, as masterminded by the Kazakhstani government, and sent to spread it worldwide.
  • In Cabin Fever: Patient Zero, Porter is an asymptomatic carrier of an otherwise highly infectious strain of a flesh-eating disease. An amoral scientist uses him as a lab rat to help find a cure so that he (the scientist) can get rich.
  • Ebola Syndrome: Kai-san contracts the eponymous sickness, but he 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 pure evil.
  • Kids: Telly is a sexually promiscuous teenage boy who doesn't know that he's HIV-positive.
  • The villains of Mission: Impossible II attempt to use Nyah as one. However, she knows it and is ready to kill herself rather than spread the disease. Luckily, Ethan gets her the cure in time.
  • The lab tech in Outbreak unwittingly infects dozens of people in a cinema after an accident in the lab in which he catches the virus. He's exposed to a blood sample from a dead man, who caught the virus from a smuggled monkey.
  • This is the central plot driver in [REC] — no one knows that the little girl doesn't actually have tonsilitis but rather something far worse.
  • In Rise of the Planet of the Apes, Hunsiker displays exactly one symptom of what will eventually be known as Simian Flu: a nosebleed (which, frankly, could be anything). He brushes it off and boards a plane, starting Simian Flu on the global pandemic that kills 90% of humanity.
  • Jack from Transporter 2 is turned into one by the villains as part of an assassination plot.

  • Invoked in Death Star. Several days after arriving at the half-built battle station, Memah is ordered to report for a medical scan and is greatly annoyed. If she'd been a Sangi Fever Sal she could have infected hundreds by now! Also this is redundant, because she got checked over before being taken to the Death Star, but that's the Empire for you, she sighs.
  • Geoffrey Allen from The Changeling Plague gets a retrovirus engineered to cure his cystic fibrosis. Since it was specifically designed for his DNA, it has only positive effects on him. Unbeknownst to him, he's also spreading to other people...
  • Ciaphas Cain: The novella "Old Soldiers Never Die" has a Tallarn regiment go from a planet experiencing an outbreak of a Chaos-tainted plague straight to Lentonia bringing the plague virus with them. However, since the Tallarns are fundamentalist even by Warhammer 40,000 standards, there's enough off-page praying and devotionals going on to block the Chaos element of the plague, so they don't get sick themselves. Unfortunately, said off-page praying and devotionals don't eliminate the Chaos element, and once the Tallarns start spreading the virus among the other military units assigned to the Lentonia uprising, the full plague starts up.
  • The novel Code Orange revolves around a teen who fears that he may be a carrier of smallpox.
  • In Down and Dirty, Croyd 'The Sleeper' wakes up with the ability to spread a new strain of the Wild Card virus; one where people who have already been transformed with Wild Card can be re-infected and changed all over again. Croyd keeps moving and spreading the virus because he has entered the paranoid stages of his meth addiction and can't trust anyone.
  • In Flight from Tomorrow, by H. Beam Piper, a future dictator escapes a revolution with a time machine to the twentieth century where he plans to be a Conqueror from the Future. However, he turns out to be a Walking Wasteland because future mankind has become acclimatized to high levels of radiation after a series of atomic wars. The military track him and carpet-bomb the valley he's in, then fill it with concrete from one mountainside to the next.
  • Miss Sneezy's 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. Yes, she is one of them.
  • In The Initiate Brother, a plague is spread to the ranks of a barbarian army by Shimeko, a former Botahist Sister who is having something of a Crisis of Faith. It's debated by other characters whether or not she filled this role knowingly or not, though the fact that she was a healer suggests that she'd have to have realized it.
  • Jacob's Ladder Trilogy: Perceval is unknowingly infected with a deadly strain of influenza and then set up to be taken prisoner by the enemy kingdom of Rule so that she'll spread it to her captors. Perceval is horrified when she later learns that she was used as an unwitting biological weapon.
  • In Clive Barker's short story "The Life of Death", an English woman ventures into a crypt that was sealed after the Black Death and becomes a carrier of the disease. She spreads it to a number of coworkers and friends, who die, and to countless others as well. She is then murdered and raped by an acquaintance who turns out to be a Serial Killer, and he becomes a carrier in her stead.
  • In "The Moral Virologist", a Mad Scientist designs a deadly disease that only kills people who have multiple sexual partners but is as contagious as the common cold or the flu. Since he has never had sex himself, being both a fundamentalist Christian and unmarried, he spreads it through the population by infecting himself and engaging in some Platonic Prostitution.
  • Naples '44, by Norman Lewis, details a plan by the Allies to send prostitutes behind German lines who were infected with severe cases of syphilis. Venereal disease had infected the Allied armies in Italy but not the German military, who ran their own brothels. Unfortunately, the mission was cancelled, and the women were released so they could continue to infect Allied soldiers.
  • Paradox Trilogy: Devi becomes one after being infected with the Stoneclaw virus. The virus works by corrupting plasmex; Devi has so little plasmex herself that it will take months to kill her, but she can still spread it to others, who die almost immediately.
  • 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.
  • 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 that there's technically three of him.
  • Thura in the Redwall book Salamandastron; probably his partner-in-crime Dingeye as well, but he never gets the chance to show symptoms.
  • Rifters Trilogy: Lenie Clark carries βehemoth back to the surface world. However, she isn't a proper example since she spreads it deliberately.
  • Sneezing Blood and the sequels have a weirdly inverted case, as the subject is symptomatic, she's just not contagious — that is, until her medicines stop working.
  • 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.
  • In Temeraire, 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.
  • The zombie plague in World War Z does not have any asymptomatic carriers, but some infected people were killed and had their organs harvested and sold on the black market. This spread the infection across the world, well past any precautions set up to stop mobile zombies. There are rumors that this was deliberate, as China — the source of the outbreak — wanted to make sure that no other country would survive unscathed while they struggled.
  • 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.

    Live-Action TV 
  • The 4400:
    • In "Carrier", Jean DeLynn Baker, an unbalanced woman who disappeared in 1999, grows spores on her hands over night. These spores release a toxin that kills everyone else in her hometown of Granite Pass, Oregon, including her parents. Jean can't control her ability, which thereafter manifests whenever she becomes angry or upset. She eventually comes to believe that she is on a Mission from God to purify humanity.
    • In "Tiny Machines", Danny takes a promicin shot and soon develops the ability to spread promicin like a plague. He himself is immune to its effects. In the following episode (and Series Finale) "The Great Leap Forward", his mother Susan is the first person to die but far from the last. When he brings her to St. Ambrose Hospital, Danny's presence causes half of the people there to die from promicin exposure while the other half survive and go on to develop abilities. NTAC and the government initially believe that the outbreak is confined to the hospital but it becomes readily apparent that it is spreading throughout Seattle like a contagion. It even reaches the NTAC building itself. Hundreds of people die within hours. By the time that the outbreak is contained eight days later, the confirmed death toll stands at 9,000.
  • The Trope Namer appears in 1000 Ways to Die. Because of her unsanitary cooking practices, she ends up infecting and killing a rich couple she was hired to cook for.
  • Dark Angel: Original Cindy's girlfriend is turned into one and eventually dies from it.
  • In an episode of ER, a staph infection spreads throughout the department, and Jeannie teams up with one of the doctors from the infectious disease department to figure out who's doing it. They both immediately know that it's from someone not washing their hands adequately after going to the bathroom but are initially flummoxed by realizing none of the infected patients have the same doctor or nurse in common. Shortly afterward, the doctor notices all the charts have the same handwriting on the top page, letting Jeannie figure out that the person doing the infecting is Jerry, the desk clerk.
  • An episode of House has an old lady volunteer at the hospital, handing out teddy bears and unwittingly spreading echovirus. She barely has any symptoms, and healthy adults are unaffected, but it's lethal to newborns.
  • The organ donor variant has also been used on Law & Order and NCIS, usually with black market human tissues and cancer.
  • Invoked by Hawkeye in M*A*S*H when he steals a jeep to go see Trapper off at the airport. He gets stopped by the MP's and lies that Radar has some kind of highly contagious disease (in reality, neurapraxia is caused by physical blows causing nerve damage) and that Hawkeye is himself an asymptomatic carrier.
  • Mike Barnes from the Midnight Caller episode "After It Happened" is HIV-positive, but he continues to have promiscuous, unprotected sex, thinking that if he doesn't acknowledge his disease, he won't die from it.
  • One first-season episode of Mission: Impossible has a village full of people who are being trained by an East Bloc government to perfectly blend in with Western society. The people all believe that they're being trained as deep-cover spies. In truth, their handlers plan to infect them all with a plague before shipping them out to their assignments. Rollin gets infected while disposing of the plague samples, and then comes into unprotected contact with the plot's leaders after getting captured. After Dan succeeds in extracting the team, they leave to find a hospital to get Rollin treated and the rest of their number tested, leaving the villains unaware that they might have caught the plague from Rollin.
  • Invoked in one episode of Murder, She Wrote, in which a character named Ellen wryly refers to herself as "the Typhoid Mary of murder" — she herself is not murdered, but people around her are, and Ellen has a tendency to be accused of involvement even though she's completely innocent. Jessica, amused, tells Ellen that she can't be "the Typhoid Mary of murder" because that's Jessica's title.
  • 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 absent-mindedly picks up and bins a dropped medical tissue, then shakes hands with a recently cured (and very beloved) patient, which gives her the infection that ultimately kills her.
    • In another episode, three people die after organ transplants because the donor unknowingly had rabies.
  • Star Trek:
    • Star Trek: The Original Series:
      • Spock in the episode "Miri", although he never actually spreads the disease he passively carries, simply because everyone else around him is infected too.
      • The episode "The Way to Eden" features Dr. Sevrin, carrier of the Synthecoccus Novae virus, who's less malicious than just crazy in the coconut. While the episode doesn't have any reported infections, this is because Federation citizens are immunized against the disease. The problem is that Sevrin wants to go live on a primitive planet whose inhabitants are susceptible to the disease and do not have any immunities. Kirk orders him to be arrested and isolated, which causes Severin's followers to get very angry.
        McCoy: All the others are clear. He doesn't have it. He's a carrier. Remember your ancient history? Typhoid Mary? He's immune, as she was, but he carries the disease and spreads it to others.
    • In the Star Trek: The Next Generation episode "I Borg", Geordi proposes using the captured Borg drone Hugh as a carrier for a computer virus meant to destroy the Borg Collective. Picard eventually decides against it on moral grounds (he comes to see Hugh as a person rather than a nameless Borg drone), for which he is roundly chewed out by Admiral Nechayev in "Descent". However, the sense of individuality that Hugh gains from his exposure to the Enterprise crew does infect his cube, which results in the Collective severing its connection (in much the same way that a computer security guy would unplug a computer actively being hacked).
    • In the backstory to the Star Trek: Voyager episode "Child's Play", the Brunali genetically engineered a child, Icheb, to carry a pathogen fatal to the Borg in the hopes of eliminating the Collective. It did succeed in killing the Borg aboard the cube that assimilated him, but it wasn't transmitted beyond it.
  • In Stargate SG-1, before the Ori gain the ability to attack Earth directly, they try infecting an SG team member with a virus. He then spreads it to the rest of his team, and once they return to Earth, they bring the disease back with them. The man who was originally infected turns out to be immune to the disease, ensuring that he doesn't die before he spreads it to as many people as possible.
  • Wonder Woman (1975): In "The Pluto File", Robert "The Falcon" Reed is an unknowing carrier of bubonic plague.
  • The X-Files:
    • Given an I'm a Humanitarian twist in "Our Town" — a town of cannibals catches Cruetzfeld-Jakob disease by serving up an infectee at a ritual banquet.
    • In "El Mundo Gira", a mutated undocumented laborer secretes an enzyme that causes anyone who touches him to be overwhelmed by lethal fungal infections within minutes.
    • "Jump the Shark" is about a biological terrorism plot based on sending out two Typhoid Marys. (Maries?) Unusually for the trope, they are both fully aware and willing participants.

  • In The Gamer's Alliance, while staying in Alent, Rhylian learns that she is a symptomless carrier of the Blood Fever and has thus infected several of her elven kin with the fatal disease throughout her journey.
    Waldheim: My lady... everything I suspected — everything I feared — has proven to be correct. It wasn't Nesa that infected Killian or Nalaen, or that poor elf in Vanna, or... probably countless others elsewhere. And now, an ever-growing number here in Alent. It was you. It... is you.
    Rhylian: No. You're wrong, Waldheim. How can this be? I've never been sick, how can...? No...
    Waldheim: I'm sorry, Rhylian. But these samples... the exact strain of the virus in Nalaen's and Killian's blood is the same one that's in your blood. I suspect you contracted the disease long ago, certainly before you left Sanae. But instead of suffering the sickness that affects most... you instead became an infectious, symptomless carrier. And I can see in your eyes that you've suspected it yourself for some time, but have been too afraid to face the truth.

    Tabletop Games 
  • Pathfinder: Whereas Paladins gain Ideal Illness Immunity, their Antipaladin Evil Counterparts become asymptomatic carriers of any disease they catch, the better to spread misery and death in their wake.
  • Red Markets: "Latents" carry the Blight that turns people into Casualties. If they're not careful, they can spread it to others, and fatal injuries will cause them to immediately become a Vector. They can result from a freak accident (a crit fail on infection rolls) or taking a drug made from the bone marrow of The Immune right after getting bitten.

  • 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 Battleborn, Beatrix's Patient Zero ability turns a targeted ally for a short time into a contagion carrier that enfeebles enemies they come in contact with but doesn't suffer any ill effects themselves. They instead have their speed buffed. When an ally isn't targeted, she uses the ability to turn herself into such a carrier instead. While it's normally a choice between making an ally or Beatrix herself into a contagion carrier, the Self Medicated augment though allows it so that both a targeted ally and she become carriers.
  • Both Yerema in Dead Island and the player characters in Dead Island: Riptide are uninfected-looking carriers.
  • Another Zombie Apocalypse example is the Dead Rising series: the plague was brought to America by orphaned children (who, in turn, were brought to America by Carlito Reyes) 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.
  • Dragon Age: Origins – Awakening brings the Messenger, an intelligent darkspawn who seems to be the only one of the bunch not obsessed with slaughter, rape, or experiments of uncertain morality. If he survives the events of the game, he disguises himself and wanders the land helping travelers in need; unfortunately, he's still a darkspawn, so those he helps sometimes become infected with the taint.
  • During the Mogloween 2011 event in DragonFable, 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. A bad situation becomes worse when one of the people Andy unwittingly mutates is needed to make the Zardbie cure...
  • The Elder Scrolls III: Morrowind: One Tribunal Temple quest has you deal with a woman blessed by the Tribunal who has caught the Corprus disease. She doesn't believe you because her "blessed" state means she isn't showing any symptoms. However, she can still spread the disease to others. You need to convince her to leave for the Corprusarium or kill her.
  • 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, not immune, as they had assumed. This is 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. They then decide the only responsible way to save themselves is reach an isolated island and live off the land.
  • 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.
    • Ditto for his NetNavi counterpart from Mega Man Network Transmission, Zero.EXE, who was a sentient virus who gained a soul. Just by existing, he spread his timeline's version of the Zero virus.
  • Metal Gear:
    • In Metal Gear Solid, Solid Snake is unknowingly infected with FOXDIE to kill the members of FOXHOUND and the ArmsTech President Kenneth Baker.
    • This becomes a plot point in Metal Gear Solid 4: Guns of the Patriots when Snake's accelerated aging and the age of the virus itself causes it to mutate; given a few more months, FOXDIE will stop caring whether its targets match its targeting parameters, and Snake will become a one-man plague. He is then infected with a second strain of the FOXDIE virus which helps cancel out the old virus and will not have enough time in Snake's body to mutate itself, as Snake is estimated to have one year left at most.
  • This is invoked by the player in Pandemic and Plague Inc.. Early on, you want people to spread your disease, but not get sick from it... not that it prevents Madagascar from shutting down everything.
  • Touhou's resident Plaguemaster Yamame Kurodani is actually a friendly youkai who doesn't use her powers without reason, but her Power Incontinence causes her to spread disease anyway. As a result, she overlaps with this trope.
  • Trauma Team: Rosalia is one of these to a degree. Even after she was killed, her blood had seeped into the ground, into the flowers, and then made the monarch butterflies become carriers of her Rosalia Virus disease. Those butterflies then spread the disease through their shed scales.

    Visual Novels 
  • Hatoful Boyfriend:
    • The "Bad Boys Love" storyline elaborates this as the backstory for Fujishiro Nageki the mourning dove, who unwittingly carried a virus harmless to birds but fast-acting and lethal to humans. Upon learning of this, Dr. Shuu experimented on him (hoping to turn him into a bioweapon against humanity), which drove Nageki to suicide.
    • In the same route, we also learn that Ryouta, who already had a slightly weak immune system and weak stomach, got unknowingly experimented on by Shuu, to make him a good carrier for the virus. He eventually implanted Nageki's liver into Ryouta, having him turn into his next Typhoid Mary.


    Western Animation 
  • In the American Dad! episode “Gift Me Liberty”, Steve technically becomes this when he acquires a handkerchief that turns out to be riddled with Conquistador Measles, and as a result infects the girls who are using him to be the “good” boyfriend and their actual dates. Only reason he didn’t get those measles is this strain doesn’t infect the sexually immature.
  • According to Family Guy, Quagmire has just about every STD known to man but doesn't show any symptoms, yet spreads them seemingly left and right.
    Quagmire: I give gonorrhea, I don't get gonorrhea, okay?
    • It's not even just every STD. He has, or is immune to, every known disease on Earth. The only thing that could get him sick was a mosquito from Africa carrying a disease that was so new it didn't have a name. That said, he specifically points out that he's a carrier of meningitis and was Patient Zero for gonorrhea.
  • In The Powerpuff Girls (1998), the Amoeba Boys catch a cold and accidentally cause a city-wide epidemic when the virus mutates within their bodies. Although they are villains (or at least try to be), they did not spread the cold on purpose, and so are this instead of plaguemasters.
  • Rocket Power: Sam is one in the episode "Typhoid Sam". He gets sick, but after throwing up, he almost instantly recovers as he built up an immunity to it. However, he ends up getting everyone else sick, right before the playoffs.
    Otto: Wait a sec... Sam's not sick cause he's always sick, and we're sick cause we're never sick? That's sick!
  • Parodied in the South Park episode "Pinkeye", in which Kenny's case of being a zombie is misdiagnosed as conjunctivitis. He goes through the episode without anyone (except Chef, eventually) noticing that he's causing the "pink eye" epidemic. It's Halloween, so they believe that he's just dressed as Edward James Olmos.

    Real Life 
  • The Trope Namer: Mary Mallon was a woman who worked as a chef in New York in the early 20th century. Everywhere she worked, her employers and the other household staff would all fall ill with typhoid fever, after which she would leave and find new employment. She was ultimately caught in 1907 and forcibly quarantined, given the nickname "Typhoid Mary" by the Journal of the American Medical Association, and revealed that she did not wash her hands before cooking. She was given the option to have her gallbladder removed, or she would have to agree to never become a cook again. She was released 3 years later and forbidden from cooking. She changed her name and went right back at it, leaving a swath of typhoid fever infections in her wake. She was arrested again in 1915 and remained in quarantine until she died 23 years later in 1938. Autopsies showed her gallbladder was full of live typhoid bacteria. Her treatment in quarantine was unusual in that other repeat offenders, including other food handlers and people who had more outbreaks to their name than her, got off with nary a reprimand and a promise not to do it again; the government threw the book at her because she explicitly refused to stop working as a cook, backing them into a corner. However, as it was never properly explained to Mary what was going on, it's a perhaps understandable reaction: She didn't show symptoms, so she didn't believe she was a carrier. Not to mention, the concept of asymptomatic transmission of typhoid was only discovered a decade or so earlier, which was unlikely to have reached an illiterate immigrant.
  • This is one of the more serious problems faced by health workers trying to contain the 2014 Ebola virus epidemic. In the early stages, the symptoms are so minor as to be barely distinguishable from the common cold, making every victim an asymptomatic carrier for the first couple of days. Most diseases eventually evolve to a point where they have a long period of little or no symptoms, as obvious symptoms and/or early death of the infected hinder the transmittal of the disease.
  • Men with many STDs and bladder infections. As of yet, there is no method for detecting HPV (the virus that causes cervical cancer) in men. About 80% of the adult population are thought to be infected, although that may have changed due to the vaccination. Gonorrhea may not cause any symptoms at all in men, but can cause infertility in women (and, we now know, in men as well), and babies born to infected women can become blind from infection to the eyes if left untreated. Even women tend to be asymptomatic more often than not. While men are more likely (although not guaranteed) to have a pus-like urethral discharge with chlamydia and gonorrhea that causes itching and burning, women may not know they have it (or had it) until they go for a routine OB/GYN exam or seek treatment for infertility, as they are less likely to have the "classic" symptoms associated with those diseases. And there are couples who have been bouncing Trichomonas back and forth for decades, without even realizing it. This is why it's important to practice safe sex: limit the number of partners you have, use a fresh condom every time (and use it correctly), wash "down there" before and after sex, and get tested (and if necessary treated) for STIs before having sex with a new partner.
  • A significant percentage of people who catch COVID-19 show few or even no symptoms. Even those who do eventually show severe symptoms will have had a lengthy period of asymptomatic contagion before then. All this leads to a disease that is spread mostly by people who, much like the Trope Namer, feel completely healthy and have no idea they've even been infected — thus the importance of universal use of precautions such as mask-wearing, social distancing, and vaccinations simply because no one can be certain they aren't infectious.
  • Most people who carry the Epstein-Barr virus (Mononucleosis) can actually be just this - to the point where almost everybody over the age of 35 has antibodies that can target the Epstein-Barr virus.
    • This is evidently true with a lot of herpes-family viruses as well. In fact, between oral herpes and genital herpes, an estimated 90% of people on Earth are carrying some strain of HSV. (It's so common, in fact, that it is not part of a standard STI panel.) And unless you are immunocompromised or in the late stages of pregnancy, it's usually not serious.
  • This can actually be the case of tuberculosis, this is called latent Tuberculosis. It's suspected of being way more common than one thinks. So much, even Edgar Allan Poe may have been one.
  • Many animals can be vectors for illnesses that do not affect them but can be deadly for other species, such as humans. Bats in particular tend to be a common vector for serious infectious viruses like Ebola, rabies, and SARS because they have extremely effective immune systems that minimize inflammatory symptoms at all times, but this in turn causes viruses within them to adapt much quicker to keep up, effectively causing bats to become a petri-dish of super-viruses.

Alternative Title(s): Asymptomatic Disease Carrier