The UNIX, Linux, permissions architecture makes it incredibly difficult to have a serious virus capable of hevy damage. Such a virus would have to run in root mode, something no one does other than for maintenance purposes. Logging out, rebooting etc. Kills all tasks, and a virus would be wiped from active memory.
It's not impossible, but it is very hard. Worm's and Trojans may be more of a threat, but only if you give them admin, or root permisions.
In windows, the OS is very pourus. There are a hundred and one loopholes easely exploited. That and curddy software like Lookout makes Windoze a virus trap.
[ October 30, 2002: Message edited by: psyjax v6.9 /Dave ]