Linux is more stable and it is more secure. Something to do with the myriad of disk partitions which act as a barrier from one part of the OS to another, making it more difficult for viruses to spread.
Another point, though, dealing with both Linux and Mac, is this: Microsoft (for better or for worse) is the "big boy on the block" and everyone wants to pick on the big guy. I believe this is a major reason why we have so many viruses targeting Microsoft OS; they're the ones everyone wants to beat.
Possibly true, but I think the motive is simpler than that. There are far more PC's in use AND PC's are more vulnerable, so PCs are the easiest and most profitable targets.
I use FreeBSD, and have not had any security problems :D
Disk partitioning has nothing to do with any difficulty for viruses to spread itself. The major feature of Linux OS is that any programs is running as an user. That user is allowed to do some things and other things not. When a virus is entering a Linux program, then will that act as the user, who is executing it. If that user happened to be root (aka superuser) then can the virus do anything it wants, but when it runs as an ordinairy user, then will it not be as powerfull. It might not even being able to execute itself.
Windows is much more benign for people who are not familiar with the internals of a computer. When you really want to profit from Linux, then must you delve into how it all works. Hence might you say that the OS of Linux means Open Source, whereas the OS of Windows means Open System.
Well if you are comparing to Windows Vista...
According to what I've read one of the main virus problems for MACs is that they can download Windows viruses, which can then infect Windows software if any is used. If none is used I'm not convinced MACs are immune from MAC malware and I'm guessing there are MAC AV programs that aren't resource hogs, so why not run one?