While there are about 14 Trojans in the wild for Mac OS X, there are no known viable viruses in the wild. There are several "proof-of-concept" viruses... all of which did not work as intended. There are no known cases of a self-replicating, self-installing, self-transmitting virus of any kind for Mac OS X.
Recently, two Symantec employees, writing on a $125 per year subscription blog claimed—just before the opening of Apple's World Wide Developers' Conference—that they had found the first Mac OS X spam-bot involving 20,000 Macs... but no one else has FOUND any of those infected Macs, nor has anyone duplicated their claimed findings. In other words the entire report was false FUD. Their own company, Symantec, still reports the trojan, the claimed method of infection, infected from 0 to 50 machines. That is true. The trojan was attached to downloads of a free iWork'09 Demo on a couple of BitTorrent sites. Those sites reported the download of the infected file was "in the dozens." The demo itself was available for easier and for free from Apple's own download servers.
“What 40 year old OS are you referring to?” . . . The Mac OS.
Driftdiver is somewhat right... because OS X is a fully POSIX™ compliant, certified UNIX™ which has its roots in the UNIX development that started in the late 1960s at Bell Labs. It was designed from the ground up as a multi-user, multi-tasking OS, with security as a primary focus. However, because of those 40 year old roots, which have undergone decades of trial by fire, OS X's underlying OS is industrial strength secure. On top of that core UNIX, Apple has put one of the most modern and intuitive user interfaces ever designed—one that Microsoft has been ripping off for a couple of decades—and has added some very sophisticated add-ons to handle graphics, sound, etc.
“There are no known cases of a self-replicating, self-installing, self-transmitting virus of any kind for Mac OS X.”
A tired old mantra that really doesn’t matter anymore. The main goal today is theft of valuable information or remote control of your PC. It is not destruction of your machine.
Those goals are easily achievable with an unsophisticated user base.
The MAC OS does reach back 40+ years although it has been updated. The Windows OS reaches back nearly as far. While MAC has a very trendy following now they have been incompetent through most of their history. Their user base has been small and made up of mostly literate users.
Microsoft won the marketing war early and nearly put MAC out of business. While Microsoft has its problems it has become the core of desktop computing. It has been tested by thousand of hackers for years and years. Yet it still remains the dominate OS for the desktop.
Developing any product to be used by 1,000,000,0000 users is far different than a product for 1,000,000 users. The complexity of the development and maintenance is unbelievably more complex.
So as much fun as is had knocking Microsoft they have done a lot of things very very very well.
And back to your worm/virus argument. As long as you can get a user to click “ok” you can do anything you want. As more unsophisticated users buy MACs you will have more machines compromised. As more machines are compromised the threat will increase.