< http://securityresponse.symantec.com/avcenter/vinfodb.html> is the searchable database. I just searched for the word Macintosh and got 2,869 documents found, top 500 by relevance.  I have obviously not read all of them to see how many reference actual threats to a Mac.
Here is what is in the defs for NAV 10 on my Mac, dated  10/5/2005 say:

33,658 Virus names found, 70,732 total virus definitions. I can then sort by type.
Hypercard Virus -- 12 (I doubt most Mac folks still use Hypercard)
Macintosh file infector--34
Macintosh Trojan Horse--19
Macintosh Worm--2
Macros--too many to count. These are cross-platform on Office documents.
PC Virus--too many to count. I don't know how many of these might be a threat under Virtual PC because I don't have it. Running a Windows AV program under VPC might be recommended for those who do but again I don't have it.

There is no breakdown listed as to how many are OS X although I recognize a few related to concept viruses or worms that have not been seen in the wild but have been mentioned online.

There are *no* viruses for OS X specifically. There are literally thousands of Word and Excel macro viruses, but you can keep them from running by enabling "macro virus protection" in preferences in those programs. (Business users, who frequently receive legitimate Word and Excel documents with embedded macros, may prefer to have an anti-viral program that can actually detect and clean a malicious macro from a document, and preserve the document.)

There are two or three Trojans/worms for OS X (not just "concepts"), but they are incredibly rare, and they aren't self-propagating, so you are unlikely to encounter them, and only then if you engage in downloading from peer to peer networks.  Trojans for OS X include Opener/Renepo, the WordInstaller Trojan, MacCowHand, and MP3/Concept. MP3/Concept does not exist in the wild as anything other than a proof-of-concept.
<http://www.sophos.com/virusinfo/analyses/maccowhanda.html>
<http://www.macintouch.com/opener02.html>
<http://securityresponse.symantec.com/avcenter/venc/data/macos.mw2004.trojan.htm>
<http://www.macworld.co.uk/news/index.cfm?NewsID=8406>
<http://www.intego.com/news/pr41.asp>
<http://www.securityfocus.com/archive/1/395107/2005-04-03/2005-04-09/0>

As for MP3/Concept, when someone posts a proof-of-concept on the Internet, my personal feeling is that it is sort of like providing a construction kit for psychopathic geeks to create malware. Thus, the mere existence of such a proof-of-concept on the Internet heralds the need for increased security.

There is no spyware for the Mac that can be disseminated via a Web site or e-mail, so it is highly unlikely that you might become infected with spyware.

There are classic viruses (for OS 8/9) that can infect Classic running under OS X, but they have become very rare because they were designed to propagate via floppy, and Macs haven't used floppies in ages. (Folks don't seem to share user-recorded CD's like they did floppies.)

So, your chances of encountering any malware at all, if you are running OS X, is minuscule. Most Mac users feel that using anti-viral software is a waste of money. I have used anti-virus software religiously for at least the last couple of decades, and its been a long while since it flagged anything other than a Windows virus that has shown up as an e-mail attachment. (Windows viruses are completely harmless to Macs.)