These are memory resident parasitic viruses. They copy themselves to DOS data area at the address 0000:0600 or 0000:0500 depending on the virus version, hook INT 21h and write themselves to the end of COM files that are executed or loaded as overlays.
When the AV.EXE or SCAN.* file is executed, "Tiny.330" deletes them and displays:
This scan program can't find me
I'm a GHOST in your machine!!
"Tiny.340" writes itself into the zero-bytes area in the COM files. The file length does not grow.
"Tiny.Fred" contains the string "Fred". This virus infects .EXE files as well as .COM, as a result these files are corrupted and halt computer on execution. Sometimes this virus formats the disk sectors.
