This entry was published at least two years ago (originally posted on August 29, 2003). Since that time the information may have become outdated or my beliefs may have changed (in general, assume a more open and liberal current viewpoint). A fuller disclaimer is available.
According to the Network Associates Virus Information Library:
Out of 71,000 known viruses…
…579 are known to affect the Macintosh.
Of those 579…
…553 are Microsoft Word macro viruses.
Of the 26 viruses that are left…
…all 26 affect only the classic Mac OS — System 9 and prior.
There are zero known Mac OS X viruses or worms.
(via The Mac Observer, via Jim)