Computer virus is a hostile program that generally replicates itself and infests other files. These malicious programs are developed by computer hackers and disgruntled programmers.
They are usually written in assembly language or high-level languages like 'C'. Some of these may be harmless as they display some messages or playa tune. But most of the viruses are highly
harmful to computer systems. The harm done to the computer system depends on the type of virus and frequency of its strike.
Viruses are quite common in DOS and Windows environments. They normally spread through floppy diskettes carrying pirated software. With microcomputers joining the Internet, the spread of virus is swift and wide through this global network.
A worm is a program that copies itself repetitively into computer memory or disk drives until the   memory or disk space is exhausted. It can cause the computer to stop functioning as it runs out of memory to load and run applications. Worm is an autonomous agent capable of propagating itself without the help of other programs. Most worms disrupt services and create problems for system management. Some worms scan passwords and other sensitive information and pass them to the attacker.
Some worms can install viruses that can cause destruction of data or damage to systems.
Origin of Computer Virus
John Von Neumann first suggested the concept of self replicating programs in 1950. A few of the programmers at AT&T started working on self-replicating programs as a game. The game involved writing code to destroy other's code in the computer memory and the one who has maximum number of lines of code wins the game. The game was kept a secret and was never released for the public. Later, Ken Thomson, co-author of UNIX encouraged programmers to write programs like the
one at AT &T. Early viruses were developed by hackers to enter other's systems.
Types of Viruses
There are thousands of viruses and several thousand variations of these viruses. Some of the commonly found computer viruses are:
1. Stone Marijuna Virus
This affects boot sector of floppy diskettes and the master boot record containing partition table on hard disks. It displays a message "Your PC is stoned".
2. The Vienna-648 Virus
This virus infects files with size larger than 10 bytes. It usually infects' .com' files and increases infected file size by 648 bytes.
3. Joshy
This virus is of Indian origin and affects boot sector of floppy diskettes and master boot record of hard disks. It becomes active on the 5th of January every year and the computer system hangs when the virus is active. If the message "Happy birth day Joshy" is typed, the system starts functioning normally.
4. Dark Avenger Virus
It infects files with size larger than 1800 bytes and is very difficult to remove. It affects' .com' and' .exe' files. It increases the size of infected files by 1800 bytes, reduces RAM by 4 KB and displays the message "Eddie Lives Somewhere in Time".
5. Stealth Viruses
Stealth viruses are hard to detect and are very dangerous. They include 512A, Holocaust, V-2000 Whale, Murphy and Michael Angelo. The Michael Angelo virus wipes out data on hard disk on Friday the 6th of March, the birthday of artist Michael Angelo.
6. Flush Virus
It is a memory-resident virus that infects' .com' and' .exe' files.
7. Jerusalem Virus
It affects' .exe' and' .com' files. It increases the size of '.exe' files by 1808 bytes and that of' .com' files by 1813 bytes.
8. Perfume
It infects '.com' file and increases its size by 765 bytes. It shoots a question and if the number 4711 (the name of a German perfume) is typed, it functions normally.