What is computer Directory?

In computing, a directory is a stuffing system cataloging structure which contains references to other computer files, and conceivably other directories. The Root Directory is the top directory in such a filesystem that does not have file or directories parent on its own.
Raying directories from root directory are sub-directories. Everyone know directories as folders.

What is a computer data cluster?

The smallest managed section of a memory that contain a file is called Cluster or File Allocation Unit.

How does Directory virus work?

The malware that inserts a virus code into a cluster and marks it as allocated in the FAT is called Directory Virus. This malware after saves the first cluster and push it to target other Files Allocation Units, pointing each file it want to infect. The directory virus load different pointers that make it look like that every file on the memory is infected by a malware when there is only one that is infected actually.

An example of directory virus is the DIR-2 Virus.

Dir-2 virus

The Dir-2 directory virus is the first DOS cluster malware. In the early 90s this virus spreaded over a the entire world globe.

When the user execute on a clean system a file that has been infected by Dir-2 , the virus writes itself to the last file allocation unit and every file on that cluster will be overwritten. After that the directory entries pointed as deleted are infected if they have the extension .EXE or .COM.

This virus was everywhere and mostly in Bulagaria, Poland, Hungary, Yugoslavia and USSR but also in the north Norway and Taiwan. The hackers of DIR-2 managed to program a little over 20 variants.
Read more on history of malware here!