Yeah, it's really sad how the virus vendors have twisted the term "virus" to mean more than what it really does. I have yet to witness or know anyone who has witnessed a "true" virus on any *NIX system. A virus would certainly be somewhat effective on a specific flavor of UNIX or Linux (the binaries have to be capable of running on the operating system in question and you can't just take Linux binaries and run them in Solaris etc).
That is, if everyone executed said virus as "root" but most UNIX systems discourage using root for normal user activities. It was a default in Lindows but I beleive they have changed that way of thinking with all the hammering they got. And some OSX people here have mentioned they always log in as root. A binary executable file (doesn't actually have to be binary) when executed by a user or administrator typically searches out other executable files and embeds itself into those executable files. So when the other executable files are executed by someone it again searches out more files to infect.
In a normal UNIX environement people don't have write access to any of the system executeable files therefore can not spread. So if it continues to be a small number of users that log on to their systems as root there really isn't much of a chance for a virus to go anywhere. And of course it would not have the ability to infect all *NIX operating systems, only the operating systems that it was compiled for.
The big difference between *NIX and DOS/Windows is in *NIX security is important (especially local/user security). In Windows it isn't (actually it is, but Microsoft doesn't understand that).
[ January 15, 2003: Message edited by: void main ]