The reason is that, for any program or a command, to gain true access the
system under linux(unix in general), would require root privelidges. Any
program/command/whatever without superuser privelidges cannot cause any
damage to a Linux box. And suppose even if there was a virus which ran
without superuser priv. what damage will it be able to cause at the most?
Damage the files owened by the user who executed the file containing the
virus. It wont be able to spread itself. Where as if
the file gets executed by the root user, than yeah, it will be able to
cause havoc.
Trojans are the only things which are common under Linux(again unix in
general) systems. Not sure about worms though(the only one I am aware of
was the Internet worm, which did cause alot of damage).
4 more info check the following link :
*www.securityfocus.com/columnists/188
Hope this answers ur question.
*www.danasoft.com/sig/linuxg33k.jpg