“Getting a Mac would have prevented it in the first place, however, since they’re essentially immune from all types of malware, including viruses.”
Not really
“What 40 year old OS are you referring to?”
The Mac OS.
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.