Please read the ENTIRE article and look at the chart. Macs ARE infected with OSX malware.
The rate for Mac infections is far lower than those for Windows, and of the infections that are Mac related, over 75% relate to the Flashback trojan (unpatched machines).
No matter what OS you opt to use - EVERYONE (yes, every single user) should have some sort of anti-malware program installed. A Mac user is better protected by the OS than the Windows user - but nonetheless - everyone should be running some sort of program. It just makes sense.
But not the 1 in 5 the misleading headline implies. IOW, the vast majority of malware on Macs is Windows malware. And this makes you feel good? For a long time, the main reason to run AV on a Mac was to be a good neighbor, filtering out Windows malware from reaching your Windows friends. This appears to still be the case.