Oldwilliam,
If you search Wikipedia for "Linux Malware" you will find a page that lists over a dozen AV applications for linux as well as 30 items under the "Virus" category. It does also mention that the danger of any of these is quite minimal. I know wikipedia isn't the most reliable of sources at times, but I think this is proof enough that viruses ARE possible.
I know that mailcious code run by a user account can only affect things that the user can affect (ie. not much), but I seem to recall their being a few privlege escalation vulnerabilities in the past (I can't remember where I saw these). Also, I think that OS X users are much more likely to blindly trust software they download and be using an account with root access than a linux user. All it takes to access system files is a single password which most users are probably used to supplying when installing various applications.
Oldwilliam,
If you search Wikipedia for "Linux Malware" you will find a page that lists over a dozen AV applications for linux as well as 30 items under the "Virus" category. It does also mention that the danger of any of these is quite minimal. I know wikipedia isn't the most reliable of sources at times, but I think this is proof enough that viruses ARE possible.
I know that mailcious code run by a user account can only affect things that the user can affect (ie. not much), but I seem to recall their being a few privlege escalation vulnerabilities in the past (I can't remember where I saw these). Also, I think that OS X users are much more likely to blindly trust software they download and be using an account with root access than a linux user. All it takes to access system files is a single password which most users are probably used to supplying when installing various applications.