Definitely Just a Cold

One of the characters has come down with something. They may be having mysterious fainting spells, or losing time, or too weak to stand, but whatever the symptoms, it's clear it's serious. If the influence of mystical power is implied, you would think they'd have cause to worry and confide in their friends about their suspicions. Or at least wonder why they're sick.

Actually, the more obvious it's not a normal illness, the less likely they'll mention it. Even if your cast includes some of the most helpful doctors or mind readers, the character won't bring it up for quite a few episodes in a row, constantly making the excuse they're just a bit under the weather. This manages to convince their unusually thick friends.

This may be used for one or more Sick Episodes. This may develop into either a Soap Opera Disease or an Incurable Cough of Death. If caused by contact with the bad guys, it may be Allergic to Evil. Often used as an excuse by a Zombie Infectee in denial. Opposite of Mistaken for Dying.

Anime & Manga

 * Joe near the end of the Gatchaman series.
 * Watanuki in XxxHolic, when his friendship with the ghost woman is making him extremely ill, to the point of coughing up blood, insists that it is "just a cold"--even when Doumeki repeatedly asks him if he's sure his symptoms don't have to do with "spirits and stuff."
 * In Deep Love believes it's a fever when it clearly isn't.

Film

 * District 9: So, you or a coworker for a multinational corporation tasked with monitoring aliens were exposed to an unknown black substance in possession of said aliens, which has caused you to: drip black from your nose, vomit black, drop fingernails, and look like you're suffering acute pneumonia. Do you A. Report your condition and have the original substance examined, B. Take a sick day to sort it out yourself, or C. Nothing at all? The answer, of course, is C. Of course, one must remember that, at the time, said infectee was trying his best to make a good impression after a recent promotion. He refuses treatment so as not to look weak or incompetent to his superiors.
 * In REC, the little girl in the movie isn't infected with latent zombiism... she merely has angina! Honest! Of course, the cast do eventually put two and two together and confront the mother about it, who is.
 * In Slither, the increasingly deforming marks, welts and swelling caused by infection by the alien worms are invariably "bug bites". Even when the bearers of said bug bites are shambling around like zombies, eating dead neighbors, or turning into amorphous tentacle monsters.
 * In Finding Neverland, Sylvia Llewelyn Davies tries to pass off her illness as a cold, but she doesn't fool her son Peter, who lashes out about being lied to.

Literature

 * In Arrow's Fall of the Heralds of Valdemar series, Herald Dirk claims his respiratory symptoms are this, after weeks of self-destructive behavior including drinking heavily, standing hopelessly outside in the rain, and generally being miserable. He does this for two reasons: he's pining with unrequited love for Herald Talia, whom he mistakenly believes to be in a Love Triangle with his best friend, Kris; and he's trying to cope with the tragic death of a trainee who was under his instruction. The fact that the person who is most suited to help him sort out this mess is Talia, the person he least wants to face in person, compounds the problem, which lasts until he collapses... from pneumonia. The Healers who attend him as he recovers spare no effort in telling him what an idiot he was being.

Live Action TV
"Woman: Look, I thought you had TB!
 * In Star Trek: Deep Space Nine's final season, the Founders contract a debilitating disease that affects only them. Odo, who had joined with the disease-ridden Head Founder, kept the fact that the disease was progressing rapidly a secret for several episodes. Bashir synthesizes a cure for the disease, disobeying orders to do so (because it could fall into the Founders' hands).
 * Played with in that at least one character figures out how sick he is, but doesn't mention it because there is no cure and she knows that he prefers to keep such things private.
 * In 24, the
 * Parodied in That Mitchell and Webb Look with a increasingly ill looking man with a severe cough who insists it's just that. Until one day his love interest shows up at the train station to meet him and he's not there. Then he shows up looking perfectly fine.

Man: TB? No, I'd have mentioned that. God! No, it's just a cough."


 * The Edwardian costume drama film it's supposed to be from is called "The Man Who Has A Cough And It's Just A Cough And He's Fine".

Video Games

 * Tracer Tong in Deus Ex develops a cough, but otherwise doesn't make too much mention of it until X51.
 * Shion of Xenosaga insists to the other characters that her fainting spells are just fatigue.
 * Colette in Tales of Symphonia kept her  secret for a long portion of the game.
 * Repeated by her in a later part of the game where she thinks she slept wrong because her neck and shoulder feel stiff. Turns out.
 * Happens in Trauma Team during a Diagnosis mission. Joshua tries to claim nothing is wrong with him. The nurse is not fooled. Neither is Gabe.
 * Also happens in an earlier Diagnosis. Jacob Tillman constantly claims nothing is wrong with him. Unlike Joshua, who eventaully admits he's not well, he fights you to the end, claiming that nothing is wrong. Turns out,

Webcomics

 * Hanna of Hanna Is Not a Boy's Name has vomited blood, become visibly weak and even passed out cold on his job and still insists he's fine.
 * Alan of The Dreamer has an ominous cough, and has sweat on his brow, but insists that he's fine.