Jafar says: Read my lips and come to grips with the reality!
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The Viruses are anthropomorphic embodiments of disease (albeit in a family-friendly manner) and the main antagonists of the Dr. Mario series as well as the spinoff Dr. Luigi - like many Mario villains they are comical in appearance and nature but are theoretically very dangerous opponents since they and their ilk would presumably spread illness across the Nintendo universe if Mario (and later Luigi) didn't keep them at bay.
Due to Dr. Mario being a puzzle-based, simplistic, game the Viruses do not speak nor have much influence but they serve as enemies along the line of famous video-game enemies such as Space Invaders or (fittingly) the original Koopa Troopas, having little backstory but being set as enemies for the hero to defeat.
Much like real-world viruses, the only way to defeat these microscopic creatures was with advanced medicine, symbolized in-game by stacking colored pills ontop of one another to destroy the virus that has the same color as the pills around it.
Mario & Luigi
Superstar Saga/Bowser's Minions
Viruses appear in the Woohoo University, attacking by chewing at Mario and Luigi's heads, which can be countered by hitting them with a hammer. When hit by either brother, the Virus changes to a different color (red turns to blue, blue turns to yellow, yellow turns to red). When all Viruses in-battle match the same color, they are instantly defeated. The red Virus is weak to thunder, the blue Virus is weak to fire, while the yellow Virus is healed by either element.
Bowser's Inside Story/Bowser Jr.'s Journey
A small blue Virus appears after knocking out all body sections of a Toothy, where it spits one out holding a small spear to attack the player.
Viruses appear during the Ultibed hunt, attacking in groups of sixteen. Like in Superstar Saga, hitting them makes them change color. Turning three or more connecting Viruses to the same color defeats them.
Super Smash Bros.
The Viruses appear as trophies in Melee and Wii U. In Brawl, the three Viruses are separate stickers. The blue Virus boosts electrical attacks, the red Virus boosts fire attacks, and the yellow Virus increases the health recovered from eating food. In Ultimate, the Viruses appear as an advanced support spirit that reduces poison attacks. Their spirit battle is a timed battle against a blue, red, and yellow Kirby on the Battlefield form of the Mario Bros. stage.
Mario Kart Arcade GP
The blue Virus appears in all three Mario Kart Arcade GP games as two items. The Wheel Virus interferes with the targeted racer by turning their wheel. It disappears after a certain time but can be passed to another racer by bumping into them. The Dizziness Virus causes the targeted racer's vision to blur. Like the Wheel Virus, it can be passed to another racer by colliding with them.