Nintendo released Dr. Mario on the NES and Game Boy on the same day in 1990, casting Mario as a doctor fighting an outbreak of red, yellow and blue viruses inside a medicine bottle. Two-coloured megavitamin capsules fall into the bottle, and the player rotates and positions them so that four or more of the same colour line up vertically or horizontally with a virus, wiping them out. Clearing every virus completes the stage, while letting capsules pile up to the neck of the bottle ends the game. Loose capsule halves fall when their partner is cleared, allowing chain reactions for players who plan ahead. Players choose the virus level, the falling speed and one of two catchy background tracks, Fever and Chill, both of which became well known. A head-to-head two-player mode sends extra capsule pieces to the opponent after combos. Its simple rules and depth made it one of Nintendo’s most enduring puzzle games.