A Virus is a germ enemy found in the Dr. Mario series. Viruses come in three colors: Red (Fever, which causes heat), Blue (Chill, which can manipulate the cold), and Yellow (Weird, which can morph people into different shapes or species). To defeat a Virus, Dr. Mario must line up three Megavitamins 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 series
Viruses first appeared in the NES and Game Boy game Dr. Mario, where they are small germs that Dr. Mario must eliminate. To do this, he uses Megavitamins, which can come in combinations of their colors (Red, Blue and Yellow), Four of these Megavitamins must be stacked on top of a Virus of the same color to eliminate it. As the difficulty increases, the number of Viruses also increase.
In the Game Boy version due to the system being incapable of displaying bright colors, Viruses appear in Black, White and Grey. The The Black Virus resembles the Red Virus, the Gray Virus resembles the Blue Virus and the White Virus resembles the Yellow Virus. Because the simple monochrome display of the Game Boy is only able to show a few shades of gray, the Black and Gray Viruses lack the black sclera that the Red and Blue Viruses had, and hence look a tad different.
Dr. Mario 64
Viruses appear again in Dr. Mario 64, and they play the same role as before. The way to beat them stays the same, too. In Flash mode, regular Viruses are immune to Megavitamins, and special flashing Viruses appear that can be defeated.
Dr. Mario Online Rx
Two new Viruses, Cyan and Magenta, appear in the Wii U game, Dr. Luigi, exclusive to the Nintendo eShop. This time they can be defeated by L-shaped Megavitamins in the mode "Operation L", and normal Megavitamins in "Virus Buster". The classic Blue, Red, and Yellow Viruses appear in the "Retro Remedy" mode, while the new viruses and the yellow, with a new name, kinds appear in the Operation L mode.
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.
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 series
Mario & Luigi: Superstar Saga
Viruses appear as enemies at Woohoo Hooniversity in Mario & Luigi: Superstar Saga. They appear in all three colors and attack in groups. When hit by Mario or Luigi, they will change into a different color. When a Red Virus is hit, it will become a Blue Virus; when a Blue Virus is hit, it will become a Yellow Virus; and when a Yellow Virus is hit, it will become a Red Virus. Once all Viruses in-battle match in color, they will be instantly destroyed, which is a callback to how they were destroyed in Dr. Mario. Viruses attack by charging at Mario or Luigi and chewing on their heads, which is countered by using the hammer. Each color is stronger than the other, with yellow being the strongest and red being the weakest.
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 appear in Mario & Luigi: Dream Team as enemies. In the field, Viruses chase Mario and Luigi upon sight. Viruses also change color whenever they are damaged (with the exception of counter-attacks); much like how they did in Mario & Luigi: Superstar Saga. The pattern they go in is from blue to yellow to red and back to blue again, repeating itself. The player can connect three or more of the same color to make them disappear, defeating the connected Viruses instantly, also like in Mario & Luigi: Superstar Saga. If a Virus is defeated, the ones behind it will move forward to the front if possible. They always appear in a group of 16, which is the most enemies seen in a battle in the real world. They are also one of the very few enemies who can inflict the Mini status on the Bros. which decreases their Attack and Defense and shrinks them.
Viruses may attack by forming two lines, one in the foreground and one in the background, and proceeding to move several steps in any direction each time. This can be dodged by jumping when appropriate. Viruses may also attack by circling the Bros. in a single file line, Mario and Luigi then having to hammer them all into a pile before they attack them after circling several times. If three of the same color land on the pile consecutively, they disappear like they would normally. If they are jumped on, they change color twice and if the hammer is used they will change color once. Also, if Mario or Luigi had the tiny status with DEF-Down they will be KO'D no matter what because the number damage is 1411. This could also give them a Game Over very easily. Red Viruses can inflict POW-Down, Yellow Viruses can inflict DEF-Down, and Blue Viruses can inflict SPD-Down.
Mario Kart series
Mario Kart Arcade GP
Nintendo Puzzle Collection
The Viruses reappeared in the Dr. Mario game (a close remake of Dr. Mario 64) of Nintendo Puzzle Collection.
Other appearances and cameos
Viruses appear as enemies in the unlockable minigame Virus Buster (also known as Germ Buster) for Brain Age 2: More Training in Minutes a Day!, which was later included in Dr. Mario Online Rx and all subsequent titles in the Brain Age series.
Viruses also make a small cameo in the obscure Super Famicom game Motoko-chan no Wonder Kitchen, appearing in a cut-scene for mayonnaise 'facts'. They also cameo in Nintendo Monopoly, appearing on one of the Coin Block (Community Chest) cards. Viruses are also referenced in Super Paper Mario when a Shayde says he was killed for an illness caused by multicolored viruses. Of all the Virus colors, the Blue Virus is the one with the most appearances and cameos.
Viruses come in three colours: Red, Yellow and Blue (Fever, Weird and Chill, respectively). In most of their appearances, all three Viruses are bipedal, wear brown shoes and white gloves. In addition to their colour, Viruses can be distinguished by their ears, horns, eyes and mouth. Blue Viruses have large, spiked ears, no horns, yellow eyes, and a mouth containing a single tooth. Yellow Viruses have two pairs of horns instead of ears, orange eyes, and no visible mouth. Red Viruses are the most distinctive, having floppy, round ears, two small horns that curve inward, yellow eyes, a mouth containing two teeth and a tongue, a spiked tail and a nose. According to the Tetris & Dr. Mario instruction manual, the Red Virus is female (making it the only one at the time). In Dr. Luigi, the new Magenta Virus also has a female design. In the NES version of Dr. Mario, when the player beats the game with the level and speed set to 20 and "Hi" respectively, a cutscene will play showing a U.F.O flying over a tree with the 3 Viruses, and then the Viruses enter the U.F.O, hinting that they might be aliens of some sort. The U.F.O. also appears before the title screen of Dr. Luigi.
Viruses are contagious; one Virus can multiply to fill an entire glass jar within seconds. According to Nintendo Comics system, each Virus has unique abilities that correspond with their names. Red Viruses can create heat, Blue Viruses can manipulate the cold, and Yellow Viruses can morph people into different shapes and species (called 'weirdness'). In the Mario and Luigi series, Viruses are able to change their colour when attacked, and are shown to wield spears. Lining up four (or three in the Mario and Luigi series) Viruses of the same colour using Megavitamins removes them without a trace.
Official profiles and statistics
Trophy information from Super Smash Bros. Melee
Sticker information from Super Smash Bros. Brawl