If everyone moved to linux tomorrow, the virus writers would move to linux, and there'd be virii a plenty. Nevermind the fact that the masses would be clueless how to patch the OS....or even when to patch the OS.
That's a mistaken concept. If you properly secure a linux distro, a virus would have a very hard time getting through. First, the virus writer has to find a way to insert his malicious code in the system from an unpriviledged user-space. That's only possible if you find an exploitable flaw (like a buffer-overflow). Given the nature of open-source, back doors and exploitable flaws are very short-lived, because there are thousands of people who develop and test the code before they are declared stable. So virus writers only have a chance with installations that are old and haven't been "patched".
As for people not knowing how to "patch" the kernel, they don't really need to know, if a distro is well thought-out. The "patch" can be installed from an online-update service, and then all the user has to do is reboot. I am alien to this principle though, since I always compile my own kernel.
But I recognize that there are people that wil never want to switch to Linux, comfortable as they are to being treated as an ignorant idiot by the software moguls, who think users are best left not being able to think for themselves and having no business as to how to run and configure their own computers (Palladium, Trusted Computing, DRM, etc..).