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2. I run anti-virus software on ALL of my Windows machines. This is to guard against Windows viruses which exploit security flaws in Windows (as opposed to the human-engineering attacks in #1 above, which are OS-independent).
3. I do not run anti-virus software on any of my Unix, Linux, or Mac machines, because #1 takes care of the trouble I could get into while using those more secure OSes.
4. At work, where I have Unix, Linux, and Mac servers which can be used to store and distribute files for Windows machines, I am beginning to implement Windows-virus software, which scans files for Windows malware even though it can't affect the server it's on -- because some Windows user might download the file and infect themselves.
5. Let us not become complacent. While I think it is unlikely that Mac OS could ever become the cesspool of malware that is now Windows' fate, there is always the possibility that somebody will eventually come up with something. The Boy Scouts' motto is "Be Prepared".