;: (By analogy with biological viruses, via SF) A cracker program that searches out other programs and "infects" them by embedding a copy of itself in them, so that they become Trojan horses. When these programs are executed, the embedded virus is executed too, thus propagating the "infection". This normally happens invisibly to the user. 

  

 These days, "[Virus]" is an umbrella term for any kind of malware, be it an actual virus, a worm, a trojan horse, or any other malicious piece of code. While [Linux] and other [Unix] derivatives are by no means immune from malware, both the sloppier system design and the popularity (and often sloppy administration) of MicrosoftWindows means that [Linux] viruses are a negligible minority. Of course, if you run [Windows] programs on [Linux] in an emulation such as [Wine] or [VMWare], that isolated environment is still susceptible, even if the system around it goes unaffected.