I have had many arguments in past decade about why it is possible and is a matter of time before viruses appear to bother Linux users. My argument has always been that any niche OS or platform will not have widespread viral attacks as they aren’t a lucrative platform for people to write viruses. The other reason why niche platforms have less of viral problems is because, the users of niche platform tend to be more educated about them or use them in a restricted way.

The moment a platform will breakout and have substantial adaption, and permits execution of user installed programs, viral incidences will be a natural follow-up. Mac or Linux won’t be an exception to this.
Found this nice article on this subject :http://www.geekzone.co.nz/foobar/6229#compact

I know a lot of people won’t be convinced and there won’t be a way to prove them wrong, unless Linux (as a general user configurable system) gains a market share of more than 30-40%. But, I guess that’ll happen sometime in future.