And I think you shoul consider that these could be propegated by unsuspecting users. Since Windows virals would not wreck havoc on a Linux box most of the time you would not notice it. But by any chance if it gets into your network and makes it to a Windows box,....you know what.
So it is advisable to use an Anti Virus software, especially if you have a mixed network environment. Open source ClamAV
is a good one for free. It has a good set of 3rd party tools
, too. The worshiped rules of digital security tell us to be paranoid. Its up to you to decide.