Linux is "virus-free" in that there are essentially no viruses for Linux in the wild, although research viruses certainly do exist. It is also "virus-free" for much the same reason as vendors give for not porting closed-source apps to it - the number of permutations is high, so the number of machines a closed-source program will run on (unless libraries are included) is relatively low.

(Vendors don't have the excuse virus-writers have of wanting to keep the binaries small and are quite capable of supplying statically-linked binaries or the shared libraries with a suitable LD_LIBRARY_PATH - and some do.)

There are other reasons, of course. "Normal" user accounts have much more limited access to the rest of the system, so making the corruption of system binaries much harder. Many distributions provide intrusion-detection software for detecting binary changes. Distributions release regular updates, which means a virus will be overwritten in a relatively short timeframe. Mandatory access controls are becoming more popular, limiting what a virus can do even if it did infiltrate a system binary. And so on.

BIOS viruses are OS-independent and so a potentially greater threat. The flash memory is usually unscanned by virus scanners, as well. However, BIOS viruses have had something like ten years to emerge and really haven't been as much of a problem as initially predicted. With the increasing popularity of flashable firmware and "intelligent" devices and daughter cards, you'd expect to see problems there too, but that's so far not happened.

Having said that, because flash content is updated far less often, requires nothing to stay in verified areas of the system, and requires there to be only a temporary exploit, I would expect to see these becoming a problem for Linux users long before true native Linux viruses themselves will be.

"Virus free" is not true at all.
However, virus (writers) have a hard(er) time in a *nix environment.
1) *nix distros don't run as admin/root by default (and you really don't need to run as admin to be productive in *nix)
2) *nix distros have secure file permissions by default. That means an "infected user" will not disrupt other users or "the system".
3) *nix approach to security is to make it the highest priority, even above usability.
4) *nix philosophy of Keep It Small and Simple makes (or at least used to make :-$ ) for modularized, cleaner, easier to maintain and more securable parts.
5) Open source and full disclosure lead to quick patches/fixes.
And I'm certainly forgetting others...

Only thing which attracts for virus makers is money ? who is going to give you money if you make virus for open source which is totally free of any cost ? and next is its open source, so everybody can see the code where is it attacking the system so its very unwise to make a virus and it get removed within seconds.

this world revolves around money...

so the only option left is to make virus for closed systems.

for anything wrong , please correct me