To be true, No! No OS on this earth can be ever be 100% immune to Viruses and Malware. But still Linux never had a widespread malware-infection as compared to Windows. Why? Let us find the reason behind this.
Some people believes that Linux still has a minimal usages share, and a Malware is aimed for mass destruction. No programmer will give his valuable time, to code day and night for such group and hence Linux is known to have little or no viruses. Had it been true, Linux should be the primary target of Malware infection because more than 90% of high end server runs on Linux today.
Destroying or Infecting one server means collapse of thousands of computer and then Linux would have been the soft target of hackers. So certainly usages share ratio is not in consideration for the above said fact.
Linux is architecturally strong and hence very much immune (not totally) to security threats. Linux is Kernel and GNU/Linux is the OS. There are hundreds of distributions of Linux. At Kernel Level they all are more or less the same but not at the OS Level.