They say a picture is worth a thousand words ...
Most relevant quote from the article: "There have been something like 100,000 viruses for MS operating systems, only about 50 for the Mac, and about 3 for commercial Unix systems. That difference isn't because of a difference in the prevalence of the machines -- there are basic architectural differences."
You can find the complete text of this very informative article, including links about computer forensics and serving as an expert witness, on GrepLaw.