Diseases are scary.
However germs are rather small. This leads to a bit of a problem. You see, scary things can make good villains, but you need to be able to see them
This has led to writers coming up with two different but functionally similar solutions to this problem. Either have the microbe get really really big or have the hero get really really small. This runs almost entirely on Artistic License - Biology
and the most extreme form of Square/Cube Law
In many cases, the wee beasties aren't disease germs, but simply single-celled organisms; favorites are amoebae and paramecia. In other cases, they're normal parts of the human body; antibodies, or blood cells.
Anime and Manga
- In Mushishi, most mushi are giant magical microbes.
Film — Live-Action
- In the Don Rosa Scrooge McDuck story "The Incredible Shrinking Tightwad", Scrooge and Donald are eventually shrunk down to microscopic size due to the effects of a malfunctioning shrinking ray, and are menaced by a horde of microbes.
- In a Super Goof story, a villain used a ray gun to enlarge animals. In the final battle, Super Goof was pitted against a van-sized amoeba.
- Evolution used a giant microbe. Technically, it's an amoeba...
- The B-Movie The Flesh Eaters features the titular micro-organisms joining together to become one large creature.
- Star Trek III: The Search for Spock has a set of microbes that get supersized thanks to the Genesis Effect. Later, they get so big that Captain Kruge gets to have a fight with one of them!
- In Fantastic Voyage, the heros had to survive being caught in the crossfire of antibodies versus bacteria, "recticular fibers" that clog the intake ports, an direct attack on Grant and Cora by antibodies, and white corpuscles.
- In some Choose Your Own Adventure books that involved some shrink-ray device, a few bad endings had microbes (e.g. paramecium, or white blood cells) attach themselves to an object which returns to normal size. In the case of an amoeba, it took Kentucky for breakfast, then Louisiana for lunch. Even at their normal microscopic size, they look very large.
- The oldest known example of this trope dates back to a French pulp adventure Une Invasion de Macrobes / The Invasion of the Macrobes published in 1909.
- The Belgian sci-fi/horror book La sortie est au fond de l'espace by Sternberg starts like this. All the microbes start growing, making the Earth uninhabitable in a series of scenes between horror and Black Comedy. For the survivors who fled in a spaceship, it goes From Bad to Worse.
- In Danny Dunn and the Smallifying Machine, the shrunken characters observe microorganisms swimming in the tiny puddle where they stop to take a drink. One of the kids mistakes them for fish at first glance, then realizes they're far too small for that.
- Galaxy of Fear: The Planet Plague gives Tash an electroscope-thing which she uses to see the virus behind The Virus. It's stated a few times to be magnifying the microbes, but is used more like magic glasses that reveal invisible things - she sees them overlaid over her normal vision and can tell at a glance if someone's covered with them. The viruses themselves seem to swim through the air after people.
- There's a Yu-Gi-Oh! card called Giant Germ that deals damage and acts like an Asteroids Monster.
- Phyrexian Living Weapons in Magic: The Gathering come into play automatically attached to a 0/0 Germ token. The germ can't survive on its own outside of the weapon, but with a few buffs to its toughness and power it's possible to beat things up with a giant Germ.
- These toys are this trope(in plush form). However not all are infectious microbes. One is yeast, a few are cells.
- Pokémon has Solosis and its evolutions, all of which are giant cells. There's also Deoxys, a giant, humanoid space-virus.
- The third boss of El Viento is a huge single-celled organism.
- Morpha from The Legend of Zelda: Ocarina of Time, whose Boss Subtitles identify it as a Giant Aquatic Amoeba.
- City of Heroes has Hamidon, giant single-celled organism and endgame raid boss.
- In Xenoblade, these show up as enemies in the Bionis' Interior, and all possess the trait of being highly resistant to physical attacks.
- Giant vaguely-microbial...things were present in earlier Final Fantasy games as generic Mooks, and seem to have been abandoned as the series became more "realistic."
- In Pikmin, one of the creatures Captain Olimar can encounter is a giant unicellular organism at least a foot across (gigantic compared to the tiny Olimar) with a watery interior and two nuclei.
- Viruses in the Mario & Luigi series. Back in Dr. Mario, these were a reasonable size (bigger than they should be, but still small enough to fit rows of them inside a medicine bottle). In Mario & Luigi: Superstar Saga and Mario & Luigi: Dream Team? They're about as big as a person is and come in armies about 16 strong at a time. They also tend to show up in rather unusual places like deserts, caves, mountains, parks and towns.
- Illusions involves guiding "amoeba-like creatures" called Gleebs to safety.
- The comic Irritability played with this once, with Chappy bragging about his victory over a giant plankton.
Chappy: It was huge!
Tatanya: Like this? (Pictures a building-sized plankton screeching "RAR!")
Chappy: ...for a plankton. (Pictures himself grasping that same plankton, now about the size of an apple, and saying "I have u now!")
- Johnny Bravo episode "The Incredible Shrinking Johnny" ends with an amoeba accidentally enlarged to giant size, which proceeds to chase Johnny and Carl down the street.
- Superman: The Animated Series featured a brief appearance by ameoba enlarged to human size in the episode that introduced the giant ape "Titano".
- The Amoeba Boys are a gang of three dimwitted crooks in The Powerpuff Girls. They're about the size of humans, and sort of float along the ground.
- SWAT Kats had a legion of gigantic purple bacteria monsters in its second episode, "The Giant Bacteria". The trouble is, they didn't much look like germs, possessing eyes, mouths and a definite torso shape with arms and legs, due to the first one having originally been an ordinary guy (more or less) who gets mutated by Mad Scientist Dr. Viper.
- Godzilla: The Series had a gigantic bacterium the Big G had to fight. It could infect other organisms with smaller versions of itself, depending on how large the organism was.
- The largest known single-celled lifeforms on the planet are amoeba-like protista called xenophyophores. They run about ten centimeters across. Small by monster standards, but positively kaiju-sized by cell standards. They are found exclusively in the deepest parts of the ocean.
- A fertilized human egg cell (and for that matter an unfertilized one) is 0.12 mm in diameter—visible to the naked eye.
- Eggs, in general, are these. The yellow part is the cell itself.
- An ostrich's egg is the largest single cell known to man.