Just to clarify, that 600,000 were infected with a Mac trojan, not Windows. And also, the end of that article was Mac ADMITTING that you CAN get a virus on a mac, however, they do what they can to be the ones keeping your computer safe instead of letting you handle your own anti-virus. Which I admit is working pretty well and they have a decent method going (for the time being), but they do admit that their potentially could be viruses for a Mac.
And in all honesty, Trojan, Malware, Virus... call it what you will but in the end it is still something unwanted on your computer doing something unwanted. The general public WOULD call this a virus, mostly because they don't know better, but the point being that the general public sees all things malicious on your computer as a virus. And lets face it, it doesn't matter what it technically is because whether its a virus or malware the fact is that its bad. Bad is bad.
I also rest my case in saying that the thread is about Macs having malware, not viruses. On top of that the exact quote from the thread starter is that 1 in 5 Macs have malware. Which is a true statement seeing as its not specifying that they are directed toward Mac. 1 in 5 have windows malware but 1 in 36 are carrying mac malware.
So those are the facts. Macs DO get viruses and the DO get malware, but in the end they mostly run on windows OS. Their is existing malware that is commonly know to exist for Macs since the 600,000 infected exploit, and these no "Virus" thats had such fame, but it is possible for Macs to get viruses, so whether it currently exists or not, it is 100% possible.