A virus is a program that replicates and inserts its code into other executable programs to spread. Viruses do not exist under Unix, and by extension Mac OS X and Linux. It is not possible for them to exist and propigate under these operating systems, because the mechanisms for them to do so cannot function.
http://www.salvagedata.com/hard-drive-recovery-terms/A worm or trojan is a script or compiled code that runs as a program itself and causes changes to the operating system. Worms must rely on documented or undocumented exploits to take control of the system, and the primary defense against these is hardened permissions, keeping up with the latest security patches for applications, and proper system configuration and maintenance.
http://packetstormsecurity.org/Unix and Linux are not obscure, they are better documentented than Windows and more fully transparent. Remember, Unix has existed since about 1971 and TCP/IP since about 1969. It is because of the obscurity and proprietary schema of Windows code that the worms and trojans can do all that they can do these days. You have no need of antivirus software if you run the computer properly, understand the configuration of the operating system and are aware of what your computer does normally.
http://www.microsoft.com/technet/sysinternals/default.mspxNorton was great back about 1988 when Peter still ran the company, but these days its usefulness is questionable... especially since it was discovered that they now use rootkit techniques to install their products. Antivirus companies make big bucks off of your fears.. big gratutious bucks for programs that you do not need anyway.
http://www.eweek.com/article2/0,1895,1910077,00.aspAnd the process of decision making that an antivirus product uses to identify threats is spelled
heuristics.
http://www.google.com/search?q=define%3Aheuristics&start=0&ie=utf-8&oe=utf-8&client=firefox-a&rls=org.mozilla:en-US:official