This is a harmless memory resident parasitic virus. It hooks INT 21h and writes itself to the end of COM and EXE files. While infecting a file the virus converts EXE files to COM format (see "Vaccina" viruses).
The virus infects the files that are executed. The virus infects the COMMAND.COM file when the first infected file is executed. This virus contains the texts:
To Nataly from Viking
In
Sometimes it also hooks INT 10h and on displaying via INT 10h it converts uppercase letters to lowercase ones, and lowercase to uppercase. While infecting a file and depending on the system timer it plays a tune.
