These are dangerous memory resident parasitic viruses. They hook INT 21h and write themselves to the middle of the COMMAND.COM file when the system is executing or opening that file. The viruses contain the internal text strings:
"Gad-Fly.629": Gad-fly
"Gad-Fly.646": The Revenger
Gad-Fly.629
While installing into the system memory, or while opening *.CAL or *.PRG files, and depending on the system timer the virus hooks INT 10h and upper-cases the symbols that are displayed. The virus also may delete the *.CAL and *.PRG files.
Gad-Fly.646
Depending on its counter and the system timer that virus erases the disk sectors.
