The Problem with the old Dosvirus are, that they can not infect PE files or exe files at all and they are not able to get controll to int13h or int21h, because Windows wouldn´t let them.
BTW DosVirusesdid you see that AVAST did not catch this one?
29a-1.zip/FILES/ZHEN7313.EXE Infected Zhengxi.7313.a
Search more Variants of these Virus and test it too! BTW2: This Virus is so bugy that it is very difficult to let it infect files, even under DOS!:)