To clarify why Linux does not get virused as much:
It's not that there aren't malicious code writers out there for Linux; it's that Linux handles permissions and security much better than Windows. If you look as how MS works, there's this nebulous thing called a registry. When a viral code is executed in MS, the registry is written to and executable files are also "virused" so that each time they are executed they reinfect the system.
This is most often possible because the local users are given local administrative rights. Do they need those rights? Probably not, but that's what MS gives them by default.
MS will also allow executable files to be written to. Think about it.... why would that need to ever happen? Why would an ececutable file need to be written to by anyone? It's and executable... it just needs to be executed.
In Linux, applications are executed by the service or user that runs the program. If the program is viral, it is only able to do damage to whatever that user has rights to. By default, only root can write to an executable file. As long as you're not spawning processes as root, you're pretty safe from a virus.
The viruses that I do know of that affect Linux boxes actually are not Linux flaws at all... they're flaws in applications that run on many Linux boxes.
I guess to sum it all up, it's difficult now days to write a poor operating system, but the people in Redmond do a pretty good job of it. With Linux, keep your applications patched and you'll be fine without any virus scanning software.