This is a virus detection. Viruses are programs that self-replicate recursively, meaning that infected systems spread the virus to other systems, which then propagate the virus further. While many viruses contain a destructive payload, it's quite common for viruses to do nothing more than spread from one system to another.
Indication of Infection
The following text strings are visible within the viral code in Terminator-3549 infected files:
Execution of infected files may result in the following message being displayed on the system monitor:
Files infected with the Terminator-3549 virus have the first 3,549 bytes overwritten by the viral code. Unless the .EXE file was originally smaller than 3,549 bytes, there is no file length increase. Files smaller than 3,549 bytes in size before infection become 3,549 bytes in length after infection. The file's date and time in the DOS disk directory listing are updated to the current system date and time of infection.
Methods of Infection
The only way to infect a computer with a file infecting virus is to execute an infected file on the computer. The infected file may come from a multitude of sources including: floppy diskettes, downloads through an online service, network, etc. Once the infected file is executed, the virus may activate.