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 throgh various means. It becomes extremely easy if an attacker/cracker (not hacker for god's sake, you idiot) gets physical access to the machine. 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 of the Empire State Building.