The JargonFile starts:

(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.

It is only paradoxical at first sight that Linux is a popular system to run Virus scanners such as ClamAV: there is no danger of infection from Windows viruses, so Linux is an ideal environment for sorting them out. The ContentScanner page describes how to protect your Windows clients from email viruses.