A virus is a computer program that is capable of self-replicating and executing copies of itself on remote systems. In this sense, the computer virus is similar to a biological virus, from whence it draws its name. While a virus by definition does not have to be malicious, the common term applies to software that is installed on users machines' without their knowledge or consent, with the intention of doing harm. Most common 'infections' experienced today are not the result of actual viruses, rather, of different forms of malware including spyware, Trojan horses, and worms. However, products termed "anti-virus" also protect against these other forms of malware.
The first known computer virus to spread in the wild (outside of a computer laboratory) was the Elk Cloner virus. Discovered in 1982, this virus affected Apple DOS 3.3 machines only. As large networks of computers had yet to be constructed, the Elk Cloner used floppy disks as its method of transmission. Computers with Microsoft operating systems, which today are seen as the most vulnerable, were not known to be infected with computer viruses until 1986. Unix-based computers have no known viruses in the wild, however there are several viruses that have been developed in laboratories to demonstrate the possibility.
The plural of "virus" is the subject of much debate. In the English language, the correct plural of "virus" is "viruses". This is because the Latin root of the word had no plural, much as the English word "air" has no plural. The most common false plural is "virii", which would under the rules of Latin pluralize the non-existent word "virius". Another common false plural is "viri", which is the plural for "vir", or "man" in Latin. Thus, "viri" translates to "men".
Programming Science Security Terminology Questions