Virus

A Virus is a germ enemy found in the Dr. Mario series. The Viruses started attacking at Mushroom Kingdom Hospital. During this time, Dr. Mario was on the scene to contain the Viruses in glass jars and eliminate them using Megavitamins.

Viruses come in three colors: red (Fever), blue (Chill), and yellow (Weird). To defeat a Virus, Dr. Mario must line up three pills of the same color next to the Virus of that color. The Virus, along with the Megavitamins, will then disappear. After clearing one jar of Viruses, Dr. Mario will move to the next one (with each jar containing increasingly more Viruses).

Dr. Mario
The three Viruses first appeared in the NES and Game Boy game Dr. Mario, where they are little germs that Dr. Mario must eliminate. To do this, he uses Megavitamins, which can come in red and blue, red and yellow, red and red, yellow and yellow, blue and yellow and blue and blue. Four of these vitamins must be stacked on top of a virus of the same color to eliminate it. As the difficulty increases, so do the number of viruses.

In the Game Boy version due to the system being incapable of displaying colours, the viruses appear in Black, White and Grey. The White virus resembles the Yellow virus, but the Black and Grey virus look different from the Red and Blue virus.

Dr. Mario 64
The viruses appears again in Dr. Mario 64, and they play the same role as before. The way to beat them stays the same too.

Dr. Mario Online Rx
Viruses also appeared in the WiiWare game Dr. Mario Online Rx, where they obviously acted like they did in Dr. Mario. They were also seen fighting Miis in the Germ Buster game.

Nintendo Comics System
The Viruses appeared as the main antagonists in Nintendo Comics System's adaptation of Dr. Mario entitled "The Doctor Is In... Over His Head". It was this appearance that named the Red Virus "Fever", the Blue Virus "Chill", and the Yellow Virus "Weird".

In the same episode, the Viruses are given special powers: Chill can manipulate the cold, Fever can control heat, and Weird can forcibly morph others into different shapes and creatures.

Club Nintendo
Two Viruses can briefly be seen inside Bowser's stomach in the Club Nintendo comic "Süße Weihnachten". They symbolize his sickness after having eaten too much.

WarioWare series
In WarioWare, Inc.: Mega Microgame$!, one of 9-Volt's microgames called Dr. Mario is based on the game Dr. Mario, and the player must defeat the Viruses using Megavitamins, just like in the game. There is also a minigame in the same game called Dr. Wario, which is a Wario version of Dr. Mario, where the Viruses look different. In WarioWare: Twisted! in the microgame Weird Science, a blue Virus makes an appearance.

Mario & Luigi: Superstar Saga
Viruses also appear in Mario & Luigi: Superstar Saga. In this game, Viruses roam the halls of Woohoo Hooniversity, Cackletta's temporary base-of-operations. Viruses appear in all three colors and attack in groups. However, in this game, Viruses can change color. In fact, when hit by Mario or Luigi, a Virus will assume a different color. When all the current Viruses in-battle match in color, they will be instantly destroyed, somewhat similar to the way they are defeated in Dr. Mario.

Mario & Luigi: Bowser's Inside Story
The Blue Virus makes a minor cameo in Mario & Luigi: Bowser's Inside Story. After knocking out all the body segments of a Toothy, the Toothy attacks by spitting out a Blue Virus holding a spear to attack the player.

In the Toadley Clinic's lobby, there is a sign on the wall that shows a blue Virus in a red circle with a "/" through it (similar to a "no smoking" or "no pets" sign).

Mario & Luigi: Dream Team
Viruses are set to appear in Mario & Luigi: Dream Team as a minor enemy.

Mario Kart Arcade GP
In Mario Kart Arcade GP, the item, Driver Virus, is a Blue Virus. In this game, the Virus has swirly eyes.

Nintendo Puzzle Collection
The Viruses reappeared in the Dr. Mario game (a close remake of Dr. Mario 64) of Nintendo Puzzle Collection.

Trivia

 * The Viruses make a cameo in the Nintendo Monopoly.
 * The Viruses in the minigame Dr. Wario appear slightly different than the ones in Dr. Mario.
 * Of all the Virus colors, the Blue Virus is the one with the most appearances, and cameos.
 * According to the Tetris & Dr. Mario instruction manual, the Red Virus is female.