First of all, its ''viruses''...not ''virii'' everyone...
Currently there are over 75,000 known viruses for the Windows platform, there are absolutely none for the Mac OS X platform...and its not because people don't want to mess with a 'fringe' OS. Unix-based OS's (like OS X) won't experience all of the problems with email-borne viruses and worms that are so prevalent in the Microsoft world. I am not saying that just because there are none, it is not possible to get a virus in OS X. Someday there may be. Sure its been almost 5 years with OS X and still none (compared to the 75,000+ for Windows), but even if one did, it would not affect any great number of computers.
This is because Mac OS X doesn't allow users to use the root account unless they first enable the option. This lessens the likelihood of email-driven viruses and worms, as most users would not even know how to enable the root account or rarely even need to. In order for a virus to attack your system it must be given root user priviliges, otherwise it will not be allowed to run. Due to the strong separation between normal users and the privileged root user, a Mac OS X user would have to be running as root to really do any damage to the system. Also, there are also no self-executing programs or files in Mac OS X. So a user would have to read the email, save the attachment, give the attachment executable permissions, and then run the executable. The user could damage his /user/home directory, but that's about it. For it to infect the entire system the steps would have to become the following: read, save, become root, give executable permissions, run. The more steps, the less likely users will bother with it, the less likely a virus infection becomes, and certainly the less likely a catastrophically spreading virus becomes.
Running as root (or Administrator) is common in the Windows world. In fact, Windows XP, supposed Microsoft's most secure desktop operating system, automatically makes the first named user of the system an Administrator, with the power to do anything he wants to the computer. Also, Windows software is either executable or not, depending on the file extension. It's easy to run executables in the Windows world, and users who get an email with a subject line like ''Check out this wicked screensaver!'' and an attachment, too often click on it without thinking first, and before they know it, a new worm has taken over their systems. Sometimes the simple act of opening the email to read it will trigger the malware.
Basically, while I would not consider any OS superior either, Mac OS X establishes a more secure footing than Microsoft Windows, one that makes it far harder for viruses to take hold in the first place, but if one does take hold, harder to damage the system, but if one succeeds in damaging the system, harder to spread to other machines and repeat the process.