Virus.DOS.Zwickau.505 or Zwickau is a virus that runs on MS-DOS.
Zwickau is a memory resident parasitic virus. It hooks INT 21h (DOS Function Dispatcher). On file creating, opening (FCB) or executing, the virus searches for .exe files in the current directory and writes itself to the end of the file.
On the 5th of any month the virus displays the message:
GRUESSE AUS ZWICKAU
Due to the language the text is in, the virus probably originated in germany, or austria.