Linux is *NOT*, contrary to popular belief, immune from viruses, or (I believe) any more Inherently resistant to them.

The mitigating factors are (IMHO)

"Traditional" viruses - mal-ware which spreads by attaching to other executables (common in the DOS days, Atari, Amiga, etc. Not so common on Windows, except for Word macro viruses)

- Linux users are generally not terribly promiscuous with binary executables
- Most binaries are either built from source, or got from RPMs, DEBs, etc from (fairly) reliable sources. These sources have been compromised in the past.
- The heterogeny of Linux systems makes a generic infection mechanism more difficult to conceive.
- Very few genuine Linux "viruses" have been discovered, still fewer in the wild.
- Better educated Linux users.

"Worms" - programs which spread by themselves, not primarily by attaching themselves to other files

- Most worms spread by either user naivety or vulnerabilities in commonly used programs
- Linux systems are (mostly) not uniform enough (like MSWin/ Outlook, or IIS) to have a single worm infect such a large proportion of hosts.
- Several fairly widespread worms are knocking around now. Nothing as big as ILOVEYOU, Codered or Slammer, but still quite common.
- Better educated Linux sysadmins.

Linux's permissions system does not provide any protection against this mal-ware. At best, it means that a virus has to attack one user at a time. So it does on Windows as Windows boxes are typically single-user.

A program that can access all my user's files on my system is as dangerous as one with root, as far as I'm concerned. Ok, so it can't trash the OS, but I don't care about that .