A virus is a physical embodiment of malicious software (or malware for short) that appends itself to a program file or a sector of a disk. Some viruses are merely annoying and can be removed with a few mouse clicks, while others can cause millions in damage and force computer owners to remove their operating systems and start all over again, or even require a new computer. While commonly thought of as a Windows-only problem, they do not as a whole discriminate between platforms, and there is no platform for which a virus cannot be coded. The reason it is thought to be a "Windows-only problem" is because since Windows has most of the market share and the monopoly of operating systems. Viruses are just one category of malware and self-replicating program, but the term "virus" is sometimes incorrectly applied to different types of malicious code.
Types of Viruses
Viruses come in many different types and are most commonly categorized according to what and how they infect.
Boot Sector Virus
Boot sector viruses were the earliest common type of virus. These viruses add their code to the boot sector of a disk, which causes the virus to be loaded into the memory whenever that disk boots. With the advent of read-only CDs for computers in the 1990's, boot sector viruses virtually disappeared. Prominent examples of boot sector viruses include Elk Cloner and Brain.
The file virus is the most common type of virus. These viruses add their code to executables, which when infected may send viral code into other executables. Prependers add their code to the beginning of the executable, while appenders place their code at the end of the executable. Also notable are space-fillers, which insert their code into the empty spaces of the executable. Non-memory-resident viruses immediately infect files in certain directories, or have ways of finding suitable files, when the already-infected file is executed, while memory-resident viruses infect files when these soon-to-be infected files are loaded into the memory. Notable file viruses include CIH and Japanese Christmas.
A multipartite virus has the ability to infect both boot sectors as well as files. The first multipartite virus was Ghostballs discovered by Friðrik Skúlason of the Icelandic FRISK Software International Corporation, makers of F-Prot antivirus.
Another way to classify viruses is how they work in the computer's memory.
Direct Action Infectors
Direct Action Infectors simply load their virus code to memory and infect whatever number of files in whatever locations it is programmed to and then immediately exits. Some may only infect one file at a time and always in a certain directory, while some try to infect all files on a computer, even on different drives and over any networks it can find. These are the most simple viruses and the easiest to create. They are the most common in terms of how many have been created, but they do not spread very far or fast.
Memory Resident Infectors
Memory Resident Infectors load themselves into the memory and stay there, continuing to infect files until the computer is shut down.
Viruses and Artificial Life
Because of their ability to replicate, computer viruses have posed questions about the nature of life. Though the question of what one can consider alive is just as much a topic of philosophy as biology, the fact that viruses possess many of the same functional aspects as biological entities suggests that they could be useful to fields of research that involve artificial life.
In Mark Ludwig's book, Computer Viruses, Artificial Life and Evolution, the author describes several possible characteristics of what constitutes a living entity, including self-reproduction, emergent behavior, metabolism, adaptability and evolution. While Ludwig concludes that viruses fall short of being living entities, he says they are still useful for the study of artificial life.
Origin of the Term "Virus"
The term "virus" used to describe self-repicating code that infects files was coined in 1983 by Dr. Leonard Adleman, the professor of Fred Cohen, the creator of the first file infecting virus. It was the first time the word had been used to describe real-life viral code. "Virus" had been used in fiction to describe similar programs. Some fictional works using the term include David Gerrold's When H.A.R.L.I.E. Was One from 1972, Michael Crichton's 1973 movie Westworld and even in a comic book, Uncanny X-Men #158 in 1982.
Misuse of the word "Virus"
The term "virus" has for some people come to describe anything that goes wrong with a computer, regardless of whether or not the program or code can spread. True viruses are pieces of code (not programs in themselves) that attach themselves to files and make those files infectious.
Virus also seems to be misused due to the fact that "virus" is not the universal term for malicious programs, but "malware" is the correct term.
The word has been popularized by the media, who report on viruses only when a virus is widespread and destructive, so it has been ingrained in the popular culture to call anything that harms a computer a virus. Many other types of malware are probably called viruses because an antivirus program detects them, and the average user comes to the seemingly logical conclusion that everything their antivirus program removes is a virus. It is not uncommon for a system administrator or virus collector to hear from a coworker or friend, "My antivirus program detected the Adware virus" when their antivirus program detects something that it describes as an "Adware" program.
Viruses and Trojans
While trojans have been called viruses, viruses can sometimes be called trojans. Sometimes operating system producers or their "fanboys" may try to call a virus a trojan in an attempt to save face, as viruses are sometimes seen as embarrassing, because it makes the system look more vulnerable to malware, possibly because of bad coding. However, many viruses are able to spread without using a single vulnerability, and a few trojans can be a result of a vulnerability.
The Plural of "Virus"
The correct English plural form of the word virus is viruses, like any other English word ending with an "s" in the singular form. As the word Virus comes from Latin, it is not uncommon to see (always on the Internet) many different mistaken plural forms of this word. Most commonly it is "virii", but "viri" has also been used.
The English word comes from the Latin "vīrus", a neuter noun. In Latin, its plural may in fact be vīri, but due to the peculiarities of Latin declension it could just as easily be vīra, vīrora or simply vīrus. Regardless of its plural form, there are no records of it ever being used in any classical texts, and it could even be said not to have a plural.
Peter Szor. The Art of Computer Virus Research and Defense. Addison Wesley, Pearson Education, Symantec Press: 2005. ISBN 0-321-30454-3
Mark Ludwig. "Computer Viruses, Artificial Life and Evolution". American Eagle Publications, Tuscon, Arizona, USA. ISBN 0-929408-07-1
Martin Overton. IBM Global Technology Services, UK, The Journey, So Far: Trends, Graphs and Statistics. 2007.09.19-21
Computer Knowledge. Computer Virus Tutorial. 2005.07.02
Tom Christiansen. What's The Plural of Virus. 1999.12.17