Viruses, and Apple, and the like
Yeah, "Elk Cloner" was the first. Well, actually, it wasn't, Creeper was, which ran on Tanex. But that's as may be.
Elk Cloner ran on Apple DOS, the operating system for the Apple II. No, not OSX. Not the operating system for the older Macs, either, that was Macintosh OS, and before that, System. Nor for the Lisa, the Mac's forerunner. No, the Apple II was a 6502 based machine, much like the Commodore 64 and all those other 8-bit greats. It was actually a very good and capable business machine for its time. But its OS bears about the same resemblance to OSX as CP/M running on an 8080 does to Windows.
In the 80s and 90s, there were a fair number of viruses for System/MacOS. But again, this OS bears little to no resemblance to current-day OSX. You simply can't run those viruses under OSX except under an emulator, and all they could hurt would be the emulated machine.
The first DOS virus, more or less, was "Brain", written for MS-DOS running on an 8086. It can still infect Windows today.
Zero virus is referring to OSX (which, although being released as OSX in 2001, goes back to NeXTStep, of 1991 vintage). It is susceptible to *zero* viruses. None. Nada. Zilch.
This doesn't mean it's immune to viruses, I'm sure that eventually it will be susceptible, it doesn't mean it's super-secure (it has its own litany of security flaws) or bug-free. But, for the moment, it's virus-free.
Makes a big difference, that does.