While viruses are much _less_ likely to strike a linux system than a Windoze system, it's not outside the realm of possibility. If you are worried about it, you can use IPTables and/or antivirus software. I believe most people who would write a virus either
a) do it to prove how much smarter they are than everyone else, or
b) want to show up _the_man_, or
c) want to cause havok, or
d) want to be famous.
each of these reasons would apply to attacking a Windoze system and a Linux System, but would be much more of a reason for attacking a Linux System. As Linux's popularity grows, I think we will begin to see Linux Viruses.