Key points appear to be:
- There are no known Mac system viruses at this time.
- There are thousands of Word and Excel viruses which can affect Mac users.
- There are a few trojans which either can or could affect Macs at this time but you have to go out of your way to install them.
- Mac spyware does exist but can only be installed with physical access to the machine.
- A Mac user could inadvertently receive and send on a Windows virus.
- Anti-virus software can detect virus-like behavior (thus possibly protecting against new as yet unknown viruses).
As a cautious professional I use anti-virus software (Intego VirusBarrier X4) with all updates installed promptly and make sure I scan all attachments. I’ve had one or two contracts which have included a clause about using anti-virus software and have been able to give the easy answer: yes I use it, rather than engaging in a prolonged discussion about whether I even need to use it.
A couple of times too, foolish Windows users have accused me of sending them a virus and I’ve been able to categorically deny it. One of them even made the accusation in spite of the fact I hadn’t even sent them any attachment at all. Of course they’d picked up the viruses through their own carelessness and via a route which had nothing at all to do with me.
I exchange a lot of attachments with a lot of people. There’s no way I’m going to open even a single one of them without an anti-virus scan.