A virus is a malicious program that passes from one computer to another in the same way as a biological virus passes from one person to another. Most viruses are written with a malicious intent, so that they may cause damage to programs and data in addition to spreading themselves. Viruses infect existing programs to alter the behavior of programs, actively destroy data, and perform actions to storage devices that render their stored data inaccessible.

Viruses attack software such as operating systems, data files, application software, and e-mails. However, viruses do not affect computer hardware.

Viruses infect the computers and spread themselves using various methods. Some viruses attach themselves to a common program, such as a popular game or word processor. When a person downloads the infected game and runs it, the attached virus program starts to execute. The virus loads itself into the memory and searches for any other program on the disk. If the virus finds any program, it modifies the program by adding the malicious code to the program. The next time this program gets executed, it infects other programs, and the cycle continues thereafter.

There are many types of viruses, such as Trojan horse, worm, boot-sector virus, and macro virus.

A Trojan horse is a malicious software program code that resembles another normal program. When a user runs the infected program, the hidden code of the Trojan horse runs as well. An example of a Trojan horse is a program that masquerades as a system logon to retrieve user names and password information, so that the writers of the Trojan horse can use them later to gain unauthorized access into the computer. Trojan horses are normally spread by e-mail attachments.

A worm is a software program that uses computer networks and security holes to replicate itself. It scans the entire network for other computers that have specific security holes. It copies itself to the new computers using the security holes, and then starts replicating from there.

A boot-sector virus places itself in the boot sector of a hard disk drive that is used to boot the computer. When the computer is re-booted, the virus automatically executes itself.

A macro virus is a macro or script that attaches itself to a file or template. When the file is loaded, the instructions of the macro or script are executed.

To prevent computers against viruses, the following actions should be taken:

• An anti-virus software that can detect, identify, and remove viruses should be installed on the computer.
• All information downloaded from the Internet or from other unknown sources must be immediately scanned for viruses.
• Any e-mail attachment that comes from unknown sources should never be opened.