@ David Kelly
"There are anti-virus programs for Macs. How many viruses do they protect against?"
I've been administrating a Macintosh workplace for about as long as Mac OS X has come out, and for most of that time we've had site license to use Sophos Antivirus.
In a half-decade or so of Sophos being deployed on fifty-odd Macs, it has caught multiple copies of a harmless Word macro virus and scored false positives on an MS Office update (not-so-false positive?) and a couple of other legitimate program installers.
On the other hand, Sophos claims it will catch this particular trojan red-handed, so it's not totally worthless.