I've been running linux for years now without any anti-virus protection of any sorts.  Just be smart about how you use it, just like any other O/S.  To begin with, one of the SmarTest things you can do is not to log in as root.  Do that, play it smart and you are safe I'd say.

I think that the main reason for a virus spreading over the entire system in a windows based system is the way their kernel is set-up and the permissions it utilizes.  Any user pretty much as enough power to execute any type of an instruction on any memory location thats in stack.  There is no protection sort of to say, okay STOP - you are executing an instruction which will change something in the system portion of the stack.

Linux, in my opinion, is far better structured it gives a 'regular' user less powers over the entire system but still enough so that daily operations can be carried out.  When you need to install something or run an executable - change permission mode to su and execute it.  After the execution is finished you are back to regular user mode.  Great.  In reality if someone was to send you a piece of code that could potentially harm your computer it would have to gain some control to the system portion of the memory/stack and most linux distributions are set up in such a way that you simply cannot do it.  If someone wants to do it - sure....they can, but logged in as a user with low permission settings you should be good.

Hope this explains it without going into too many details of an o/s is structured and how instructions are carried out.

Excellent posts guys!!!! You guys indeed make a lot of sense about the whole idea of virus attack and vulnerabilities associated with Windows etc., I think you did make a lot of sense here and that is the main reason i'm a big fan of Unix like systems rather than windows...
I appreciate you all for responding promptly with great information..