> I've never seen an anti-virus program for Linux, and I always
> assumed that it was because of Linux's natural resistance.

There are a few AV packages for Linux, clamav is useful because it can use
various addins to scan such things as:
- The usual file/archive scanning
- On Access file/directory scanning
- Incoming POP mail
- SMB share scanning
- Apache module to scan web content.

http://www.clamav.net/
But it can be a bit slow when scanning large volumes of files

F-Prot are reasonably well known commercial organisation
http://www.f-prot.com/products/home_use/linux/

Their software does not support the range of plugins/abilities that clamav
has, but it is extremely easy to install and its scanning is much faster.

Panda AV is again easier to install, faster to scan but has less options
than clamav with regard to possible uses.
http://www.pandasoftware.com/download/linux/linux.asp