Richard Stallman is explaining in this video, why the Linux operating system is virus-free, essentially the main reason is the lower market penetration of Linux; compared to Windows which is used by just about every user and business in the world and therefore is a greater and easier target due to the lower security and more susceptibility to the attacks that would not work on a Linux system due to the fact that most Linux systems have security systems like Novell AppArmor or NSA Selinux that gives good protection against many forms of malware that would try and attack the system once installed.
NSA Selinux will stop malware programs running the instant they try and execute as they most likely would try and access areas of memory that are unusual for most Linux programs. Richard Stallman is the father of the GNU Free Software movement and he is a good spokesperson for freedom as I am and he does not like Binary blobs preferring free Open Source software. I am using Intel graphics for this reason as the Linux kernel includes drivers for this hardware without having to install a binary driver like Nvidia.
I have just added some Linux pdf files here: http://www.securitronlinux.com/linux/lc/. These will be a very useful resource for those who are using Linux and want to learn the command line and how to use various programs and utilities.