What are computer viruses and how do they work?

A computer virus is nothing but a programme that is built to “infect” other programs by making certain changes in that programme that render it unable to perform its designated function.

During this procedure of ‘infection’, the virus also self perpetuates, in that it makes many copies of itself, so that it can then further infect other programmes, much like the viral infections that affect human beings.

Surprisingly, there is so much similarity between the tow, in that during the replication process the copies are made exactly like the original, with no posttranslational modifications.
Depending on what the virus was created to do, it can either delete the programmes it infects, or it can make it completely useless, and may itself turn it into an ‘infecting’ programme also.
Major categories of viruses are
1 Parasitic virus:  these are the most common form of virus, it attaches itself to executable files and replicates when the infected program is executed.
2 Memory-resident virus: this virus attacks the main cache memory of the system, and gains access to all the programmes that are executed.
3 Boot sector virus: this virus infects the boot segment, and then infects all the files that are executed from the disc
4 Stealth virus: like the stealth machine, is virtually immune from anti virus softwares
5 Polymorphic virus: after each infection, the virus changes its character, thus making cleaning or treatment very difficult
6 Metamorphic virus: unlike the previous, the virus completely changes itself after every infection. .
In the lifetime of the virus, a number of phases have been identified that allow the identification and possible clean up of the virus.
1 Dormant phase: virus is not attacking in this phase. It waits for a specific trigger for which it has been programmed. It may be a particular date or a specific programme that is required. These are specific viruses, while not all viruses have been programmed to follow this phase
2  Propagation phase: virus makes multiple copies of itself to infect many files at the same time. Thus the virus may enter the files or the system areas itself.
3 Triggering phase: the virus gets its signal to become activated. Having received its signal, it then assumes a position from which it can attack easily.
4 Execution phase: this is the final step, when the virus actually performs the step it was made to do. Thus the virus may delete the file, make the programme corrupt, make multiple copies of itself, or whatever it was programmed to perform.
Viruses are operating system specific and some even only attack specific hardwares. For example the linux operating system has not yet faced a virus attack, while the microsoft system is continuously batting many virus attacks, even the apple OS is not immune from these attacks
Another identity is a worm. This is a program that can replicate itself (like a virus), but it has the additional ability to send copies from one computer to another computer across network connections, via e-mails or VPN systems
Referances
1 Heidari M. Malicious Codes in Depth. Accessed from  www.securitydocs.com. on 13 June, 2008
2 Brain M. How computer viruses worl. Accessed from www.howstuffworks.com/virus.htm on 13 June, 2008