Finally decided to install an AV app, namely Sophos, which offers a free home edition for the Mac. This version is 8.0.2.
It immediately downloaded over 11 MB worth of 'updates' (which I presume are the honking big list of virus/worm/Trojan descriptions available to date). Sophos claims to be protecting against 3,479,000+ threats at the moment.
Now it's scanning ca 2 million items on all my local drives (including backup); this is likely to take 8 hours. So far no 'threats'.*

* And it just now finished scanning everything — ca 5.5 hours — and found nothing nasty to report.

I probably should have dropped this into THE CYBER-SECURITY THREAD in the Lounge where most of the discussion is taking place; but this is specific to a brand, so I dropped it here. If a moderator thinks it needs a different 'home', I've got no objection.

Sophos has a good reputation, but don't get too comfortable and let your guard down. As Tacit has pointed out on several occasions more and more of the new malware is undetectable by any anti-virus software and no malware can be detected unless it has been identified and its code "signature" added to the virus signature database on your Mac.

Good point. Never have, never will be (too) complacent.
On my older system I used Norton AV, but that apparently became a liability in Mac OS X.
The local university's IT services uses Sophos AV to cover their users' butts.

That could account for its reputation as "bloatware". I've read more positive things about ClamXV than Sophos, but have used neither.

OS10.5.8 imac 4Gb ram
I have used sophos but it really slowed down my computer. I googled ClamXV and I got positive reports. I have been using it for about 3 months now and a good feature that you can put whatever you like in the sidebar and scan that item.
Particularly good for new downloads, & mail.
jaybass