One in five macs where people chose to install antivirus software have (inactive) Windows malware.
Which is a bit like saying "one in five cars brought to the mechanic get serviced for something." The survey is skewed due to the sample group - most Mac users never install any anti-virus software.
The only places I've seen it installed are on computers in corporate environments where there are already viruses being passed around commonly via email attachment, USB stick, and network drives. These places install antivirus on Macs so users don't forward a virus to Windows users - and it sounds like from this survey, that's with good reason.
Apple's Mail software (and Microsoft's Outlook for Mac) cache attachments locally on the user's disk, so it's very easy to "have" malware and viruses if you just receive email (even without opening it).
It's a bit ridiculous to claim they are "infected" however, and again, the sample group is not really representative. That said, I don't think Macs are in any way immune from viruses. Apple's iOS-like sandboxing and signed-app requirements would likely help OS X considerably in this regard, but of course every decision that increases security by removing control from the user also infuriates free/open software proponents and hackers. Think of jailbreaking iOS and how Apple patches security holes - this is maddening for people who want to jailbreak, but is ultimately an attempt to fix a potential infection vector.