Linux has nothing to worry about for the moment since it's still too unpopular to be a viable target, but you can be damn sure that if it ever reaches anything near Windows' current market share, there will be a shit-ton of viruses for it..
Lulz
what do you think the web runs on... Go check out netcraft for a moment and see the number of 24/7 webfacing servers there are
Then go learn about true mult-user operating system with ACL and THEN you will realised that Linux is not as susceptible to virus's as OSX/Windows
Yes a virus can be written (shite I can knock together a bashscript that can do some damage) BUT 1) user must explicitly run it 2) bitch to spread
Only way "Linux" would go pearshaped w.r.t. virus's if some asshat proprietory devhouse knocked out shite (I mean OSX built on BSD and they have made a joke of BSD's security) BUT even still alot more of the core of linux is open for many eyes to provent that. EVEN then run a hardened toolchain system and nothing is getting in, even a vuln in a closed-source app would get terminated by the kernel and the userland the moment it did something outside its sandbox that everything is wrapped in
--edit-- just incase you don't know what "hardened" is
http://www.gentoo.org/proj/en/hardened/hardened-toolchain.xml
this is out of date (gcc-4.3.1 is hardened now)