The problem is people sending infected documents to others via non-Windows machines, either through e-mail (forwarding or Sendmail) or file transfer (either sending files or hosting a file server).
That's why OS X Server comes with ClamAV (Open source AV) built in which integrates with Sendmail. That's why McAfee, Symantec, and other vendors make Mac OS X versions of their product
.
I prefer McAfee's solution:
http://www.mcafee.com/us/small/produ...ops/virex.html