Sophos’ Naked Security blog:

One in every five Mac computers is harbouring some kind of malware, a new study from the experts at Sophos has revealed.

The key phrase here is “some kind of malware.” Sophos is including Windows malware, sent to you by e-mail, for example, by unwitting Windows users. That stuff will never give OS X a problem.

How many times have you received a spam e-mail or message from a Windows using friend (or total stranger) with a strange file attachment?

Get rid of it, obviously, but you’re using a Mac for a reason – and one of them may be so you don’t have to deal with Windows and its problems.

What’s more, Sophos’ thesis is predicated on assumptions made using data retrieved from “Mac computers which have recently downloaded Sophos’s free Mac anti-virus software.” Mac users who have downloaded anti-virus software are a self-selecting group if ever I heard one. One in five of those Macs may have some kind of malware on them, but I’d be very surprised if that was the same as the public at large.

Mac users don’t routinely use anti-virus software. I expect that many of them are downloading Sophos’ product with the expectation that they’re going to find problems.

Sophos is simply trying to drum up interest in its own products – the blog post links back to its anti-virus software for Mac, which is indeed free, but is also quite capable of building a captive audience for other products in the future.