AIDS_II is a companion virus. Companion viruses do not actually infect executables, but make copies of their viral code in hidden .COM files. These .COM files have the same filenames as existing .EXE files. DOS, when not given a specific extension to execute, will launch .COM files first, thus putting the virus into memory. Once the virus is in memory, it runs the desired file. The original .EXE file remains unchanged throughout. The virus may play music and display the following message:

Your computer is infected with AIDS VIRUS II