This is a claim almost as old as time itself… well UnixTime. It’s spread about by both Linux and Mac partisans (edit: and of course, Apple) as proof that using Windows is akin to living in a leper colony. You don’t see a citation for it. That’s because it is BS. It comes about from doing things like counting every slight modification as a separate virus and trojan and worm and through counting every individual Symantec signature (which not only counts the changes as above, but also legit-but-questionable software like commercial keylogging and monitoring software like the type the boss who distrusts you just installed on your workstation).
Of course, the numbers are further goosed by counting every variation of every malware program that affected any version of Windows. Those Windows 3.1 and 95 virus are never removed from the counts, even if they cannot harm a modern Windows computer.
If you used the same standards to Apple computers, MacDefender would be about a dozen viruses on its own, and we’d be counting the infections that felled System 7 or Mac OS 8.0 too.