Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
GNU/Linux systems are immune to viruses is anything but myth. Viruses, malware are programs with destructive intentions and can be installed on any machine, if an attacker/cracker (not hacker for god's sake, you idiot) has physical access to it. You can install a malicious code on your own computer if you want. No one can stop you from deleting all your files, and throwing your system from 20th floor or Empire State Building.
But can a cracker do it?
Everything boils down to the level of security. GNU/Linux systems are secure by design so the vulnerability rate is million times lower than that of insecure by design systems like Windows. When it comes to core system, fixes don't wait for Tuesdays, they come immediately.
After this background, I restate GNU/Linux systems are 'almost' immune to attacks. If Windows is like a bicycle, GNU/Linux is like an armored tank.