Perfect Health

Characters never sneeze, cough, belch, blow their noses or in any way display less than perfect health unless it is central to the plot (as in Annoying Patient or Nurse with Good Intentions). A simple cough might warrant a subplot to handle the ramifications, if it is not itself a symptom of terminal disease. If ever such symptoms have no plot significance, it's probably because the actor is actually sick. However, they'll usually try to work around that.

Occasionally subverted when it turns out that the fact the character has perfect health is significant -- they may have a superpower, or be a robot.

Sort of a medical incarnation of The Law of Conservation of Detail. Compare: Soap Opera Disease.

Anime and Manga

 * Shiori, from Kanon, initially hand waves her skipping of classes and lack of energy as "a cold". While this excuse rapidly wears thin, the heartbreaking revelation is somewhat spoiled by the audience being well aware it could only be a fatal condition.
 * Ohno's catching of a cold on Genshiken is what forces Saki to perform cosplay. Much to the latter's regret.
 * In One Piece, Mr. 4's weapon, a gun which can become a dog by the name of Lassoo, has a cold during the fight for no particular reason. It mostly causes a comic effect when his sneezes also fire bombs. This is a turning point twice in the fight, when Chopper forces the dog to sneeze bombs into the tunnels used by Ms. Merry Christmas, and when a knocked out Lassoo sneezes one last ball which blows up him and his owners.
 * Also, Nami's falling ill was the central plot of the Drum Arc, where it was also mentioned that Luffy, Sanji, and Usopp have never been sick before.
 * Averted and Played for Laughs in Cowboy Bebop: Knockin' On Heaven's Door where a character sneezes on a few occasions - due to his allergies - amidst an unknown outbreak.

Comic Books

 * Spider-Man occasionally catches a cold from staying out too long in the rain wearing nothing but his costume. He rarely has the luxury of staying in bed until he gets better.
 * That's because he's The Woobie. Spidey's had more colds than all other superheroes combined. Even the women, who never dress for cold weather.
 * Averted in Cerebus the Aardvark (also notorious for a four-page Nobody Poops aversion showing him urinating); Cerebus has a cold at least once but it isn't a big deal, plot-wise.
 * Averted in James Robinson's Starman from DC Comics. The dad, the original Starman, far older in mind than body, has a couple moments of coughing. In the odd way of narrating Robinson tends to do, dad reveals it was a cough, nothing else, not an incurable old man disease. And indeed it wasn't.

Film

 * Averted in ~Schindler's List~, in which sadistic camp commandant Amon Goeth does appear to have a cold, and doesn't drop dead of consumption at the end of the film.
 * It's to set up the mind-bending irony of an extermination camp commander telling one of the inmates not to get too close to him so she won't catch his cold.
 * David Dunn's perfect health is a key plot point in Unbreakable.
 * Also averted in 12 Angry Men. Juror #10 has a head cold. It's not a plot point or anything. He just has a head cold.

Literature

 * David Sedaris expresses his annoyance with this trope in one of his essays concerning The End of the Affair. When the lead character coughs it's an indicator that she will be dead in 20 minutes; "It might have been different had Julianne Moore suddenly started bleeding from the eyes, but coughing, in and of itself, is fairly pedestrian."

Live Action TV

 * In Stargate Atlantis, Sheppard is shown to have a mild cold at the beginning of the episode "Irresistible". This turns out to be of use to him as it makes him immune to the airborne chemical that has the rest of Atlantis under its spell. This of course brings up the question of just what would have happened if he hadn't had that convenient cold. Obviously, the Ancients must be watching over him.
 * A subversion: in one episode, a character's abnormally perfect health (as in, she wasn't just not sick, it seemed she'd never been sick ever) is cause for suspicion.
 * Avoided in the original Stargate movie (and early first season of Stargate SG-1) in which Daniel Jackson is stated to have allergies and he sneezes constantly.
 * He mentioned once that he always sneezes when he travels. The movie-based novels turn this into the manifestation of a phobia caused by a childhood tragedy. Every incarnation of Dr. Jackson is The Woobie.
 * Daniel Jackson also missed an episode because he had to have his appendix removed. Originally he was going to be in that episode, except Michael Shanks had to have HIS appendix removed and the writers apparently weren't feeling especially creative.
 * In Smallville, Lex Luthor has the meteor-granted power of a superhuman immune system. Being Superman, so does Clark (the one time he gets sick, it's because he went into the Phantom Zone; however this illness causes him to develop his super-breath).
 * Incidentally, Lex's super-immunity is also used to Hand Wave his baldness.
 * Contradicted in the "Killed by Death" episode, where Buffy has a severe flu which is not directly life-threatening, but nearly makes her lose a fight. Her high temperature does make her able to see an otherwise invisible killer though.
 * Played straight most of the rest of the time, though. And lampshaded and justified at the same time, within the same episode when it's mentioned that Buffy very rarely gets sick; the same magic that gives her super-strength and abnormal reflexes also boosts her immune system to increase the speed at which she heals.
 * In ~iCarly~ a minor character named Jeremy is a subversion of this trope.
 * More like an almost perfect inversion. He sneezes every few seconds, all day, every day (at least when he appears).
 * Subverted on House where the main character occasionally has a cold, a headache or some other minor health issue without it impacting the plot.
 * Parodied in a Mitchell and Webb sketch with The Man Who Has A Cough And It's Just A Cough And He's Fine.
 * Illya Kuryakin catches a cold about halfway through the The Man from U.N.C.L.E. episode "The King of Diamonds Affair." ("I've been fighting a headache and a cold. Personally, I prefer THRUSH.") It's not important to the plot in any way, and it doesn't impede his later Big Damn Heroes rescue of Napoleon Solo; its only apparent purpose is to make him miserable. Russian Guy Suffers Most...
 * Played for Laughs in the Babylon 5 TV movie River Of Souls. A Soul Hunter announces his imminent arrival at Babylon 5, and the command staff remarks that Soul Hunters only show up when someone is about to die. Zack Allen starts coughing, and is alarmed to see everybody staring at him as if he is about to keel over.
 * On a few occasions, we see scenes from the future where Londo, soon before his death, is ridden by a constant cough. Once this is established, we occasionally see Londo coughing in the present to remind the audience of his fate. Mind you, his death is via strangulation, not as a result of his illness.
 * In the "Sontaran Strategem"/"Poison Sky" two-parter of Doctor Who, Donna uses her Magical Temp skills to deduce that no one in the factory has ever been sick, which becomes the first sign that something is quite wrong.

Video Games

 * Sonic in Sonic and The Secret Rings has a cold in the very first cutscene and wishes for handkerchiefs. The cold is never referenced for the rest of the game but his first wish is referenced at the very end when he wishes for a mountain of them for Shahra due to her tears.

Web Comics

 * Not only does the titular Reality Warper in Minus say that she's never been sick, but she doesn't seem to know what sickness is. When her friend explained that she had been absent from school because she was sick, minus made herself sick to see what it was like... and threw up. She then assumes that her friend likes to throw up.

Western Animation

 * Carl Wheezer of The Adventures of Jimmy Neutron is an inversion of this.
 * Macy of As Told by Ginger is an almost perfect inversion of this.
 * Averted twice in South Park. Once in "The Snuke," where Kyle has a cold, and once in "A Very Crappy Christmas," where Cartman has a cold.
 * Averted in one episode of Chalk Zone, in which Rudy has the flu, and is home sick in bed with a fever, in the middle of July.