Biologically, a virus is a microscopic parasite. It can only reproduce by infection of a host cell. It is unable to replicate on its own. Similarly, a computer virus is a program that infects computer files by making copies of itself in software, hardware, or over networks. Like a virus, a computer virus is unable to self-generate. It needs a host program’s execution to spread itself, hence it is often attached an existing file. Only when the infected program is executed, the virus is able to write itself into memory and find the opportunity to infect other computer files on the host or other potential hosts to infect. It is important to note the difference between a computer virus, a worm and a “Trojan”. A worm can more fittingly be compared to bacteria. It has the ability to replicate itself over a network. A Trojan, like the Trojan Horse in Greek legend, often appears to be performing some useful functions while it is covertly performing possibly harmful acts.
On the simplest scale, a computer virus has two functions: a. to replicate and propagate itself, b. to execute the implemented codes that do the damage.
The first known virus that spread to a considerable extent was Elk Cloner, circa 1982. Interestingly, it was written by a then 15-year-old high school student. The target of the Elk Cloner virus was Apple II operating systems, stored on floppy disks. Elk Cloner’s replicating mechanism was relatively primitive comparing to present ones. When an computer is booted from an infected floppy, the virus would automatically make a hidden copy of itself onto the system; it does not affect the normal operation of the system, but passes itself onto uninfected disks accessed on the system. The virus affected the boot sector of the infected system. The virus is launched every time a system booted up from an infected floppy disk (Operating systems were often stored on floppy disks back in the days), but is rather harmless. All it did no more than displaying an annoying message on every 50th boot-up.
Since then, the viruses had evolved to be much more malicious than displaying an annoying little poem. It could alter, or even completely erase information on software or hard-drive. Everyone has heard of worms like Mydoom and ILOVEYOU, which made major headlines. It remains a major concern in that tens of thousands of viruses are floating on the Internet today.
References: Aiyappa, Krithi. Virus, worm, what’s the difference? http://www.ciol.com/content/home/techie/100072501.asp
Viruslist.com. History of Malware. http://www.viruslist.com/en/viruses/encyclopedia?chapter=153311030