According to the Tetris and Dr. Mario instruction manual for the SNES, the Red Virus is female. This information is absent from the normal Dr. Mario instruction manual for the NES.
Contributed by JelloBoyInc
If the player destroys four or more viruses at once, a hidden Super Mario Bros. theme will play. This only works on the Game Boy version.
Contributed by xaxdxaxmx
The 1989 prototype of Dr. Mario is the earliest known build of the game, and at this point in development was called 'Virus'. The prototype has numerous differences from the final version of the game.
Contributed by Tobu_