what is computer virus? | computer virus

 Computer Virus

Computer virus is a small block of coded instruction that obtains control of a PC's CPU and directs it to perform unusual and often destructive actions. Virus also called perverse software is a program which knows how to copy itself and attaches itself to other programs which further spread the infection. Antivirus is the method to get rid of perverse software of the viruses.
Viruses are classified in the following types:

1. Trojan Horse
2. Time and Logic Bombs
3. Melissa and Ska Virus pertaining to various MS Office files.

Trojan Horse

A Trojan Horse is a program that invades a computer system by secretly attaching itself to a valid program downloaded into the computer. It may be used to locate password information, on it may alter an exciting program to make it easier to gain access to it. A virus is called a Trojan Horse that continues to infect program over and over again. It also pretends to do something useful or interesting. But when run, it provides some harmful effects like scrambling FAT (File Allocation Table) or formatting the hard disk.

Time and Logic Bombs

A Time and Logic Bombs is a program that destroy data. For example, it may reformat the hard disk or randomly insert garbage into data files. A time bomb as the name suggests, is triggered by an event. It can format the hard disk on a given date or slow down computer every Friday or make a ball bounce around the screen.
A logic bomb may be brought into a personal computer by downloading a public-domain program that has been tampered with.

Melissa and SKA Viruses

Melissa virus attacks MS Word documents and spreads very fast. This virus affects word documents by installing itself to normal.doc and disabling the macro protection message and infects all the word files opened or created subsequently. The message generated by the virus says "Here is a document you asked for.....don't show anyone else".
A new virus called Melissa spread around the world rapidly, infecting many computers and bringing down networks. It was a Word Macro Virus and used Outlook to send infected documents across the Internet.

Melissa has two types of impact. First the initial snowball effect and then the ripple effect. If the end users open the attachment that causes the problems, corporate systems are flooded with resource-hogging e-mails, in exponentially increasing numbers. As the snowball gets momentum, more e-mail servers are affected. There virus called Ska increases the size of the file and changes the extension of the files to .ska. These viruses delete the MS Office Document files or change their contents. Happygg also known as Ska or I-Worm is a computer Worm for Windows.