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What is a virus?

A virus is a program that can infect or contaminate other programs and is often detrimental to computer data or system integrity. Viruses are most frequently spread by opening infected email attachments. The virus can then send itself to everyone listed in the recipient's email address book. Some viruses are spread through Internet downloads, while others are spread through infected disks. There are thousands and thousands of known viruses, and new virus strains continue to show up regularly. The rate of virus infection is rapidly increasing.

History of computer viruses

A program called "Elk Cloner" is credited with being the first computer virus to appear "in the wild" -- that is, outside the single computer or lab where it was created. Written in 1982 by Rich Skrenta, it attached itself to the Apple DOS 3.3 operating system and spread by floppy disk. This virus was originally a joke, created by the high school student and put onto a game. The game was set to play, but release the virus on the 50th time of starting the game. Only this time, instead of playing the game, it would change to a blank screen that read a poem about the virus named Elk Cloner. The computer would then be infected.

The first PC virus was a boot sector virus called (c)Brain, created in 1986 by two brothers, Basit and Amjad Farooq Alvi, operating out of Lahore, Pakistan. The brothers reportedly created the virus to deter pirated copies of software they had written. However, analysts have claimed that the Ashar virus, a variant of Brain, possibly predated it based on code within the virus.
Before computer networks became widespread, most viruses spread on removable media, particularly floppy disks. In the early days of the personal computer, many users regularly exchanged information and programs on floppies. Some viruses spread by infecting programs stored on these disks, while others installed themselves into the disk boot sector, ensuring that they would be run when the user booted the computer from the disk.

Traditional computer viruses emerged in the 1980s, driven by the spread of personal computers and the resultant increase in BBS and modem use, and software sharing. Bulletin board driven software sharing contributed directly to the spread of Trojan horse programs, and viruses were written to infect popularly traded software. Shareware and bootleg software were equally common vectors for viruses on BBS's. Within the "pirate scene" of hobbyists trading illicit copies of commercial software, traders in a hurry to obtain the latest applications and games were easy targets for viruses.

Since the mid-1990s, macro viruses have become common. Most of these viruses are written in the scripting languages for Microsoft programs such as Word and Excel. These viruses spread in Microsoft Office by infecting documents and spreadsheets. Since Word and Excel were also available for Mac OS, most of these viruses were able to spread on Macintosh computers as well. Most of these viruses did not have the ability to send infected e-mail. Those viruses which did spread through e-mail took advantage of the Microsoft Outlook COM interface.

Categories of viruses

File-Infecting Viruses
This is the most common type of virus. A file-infecting virus attaches itself to an executable program file by adding its own code to the file. An executable program file can be run and has an .EXE extension. The virus code is usually added in such a way that it escapes detection. When the infected file is run, the virus can attach itself to other executable files. Files infected by this type of virus usually have a .COM, .EXE, or .SYS extension. Damage to data can occur when the virus is triggered. A virus can be triggered when an infected file is executed, or when a particular environment setting is met (such as a specific system date).

Boot-Sector Viruses
When a computer boots (or starts), it looks to the boot sector of the hard disk before loading the operating system or any other startup files. A boot-sector virus is designed to replace the information in the hard disk's boot sectors with its own code. When a computer is infected with a boot-sector virus, the virus' code is read into memory before anything else. Once the virus is in memory, it can replicate itself onto any other disks that are used in the infected computer.
Trojan Horse Programs

Trojan horses
As a legitimate program such as a game or utility, but once it is executed, it can destroy or scramble data. A Trojan horse program can contain viruses, but is not a virus itself.

Worms
A worm is an independent program that replicates itself, crawling from machine to machine across network connections. It often clogs networks as it spreads, often via e-mail.

How do viruses spread?

Viruses are most frequently spread by opening infected email attachments. The virus can then send itself to everyone listed in the recipient's email address book. Some viruses are spread through Internet downloads, while others are spread through infected disks.

How can i prevent computer viruses?

You must buy and run quality software on all your computers to protect you from viruses, spyware, and other potentially harmful applications. There is no budget solution, you must do the following or it is likely your computer will be hit by a virus and your work/data will be lost:

Get good anti virus software e.g. one of the below.
Regularly update your 'virus definitions' e.g. once per week.
Email Attachments: BEWARE OF ATTACHMENTS. Do not open email attachments that you are not expecting. Viruses come with some very nasty messages to trick you into opening the attachement e.g. "Your email account has been cancelled, see attachment for details". Even worse, the virus looks like it comes from an email address you recognise e.g. from admin@yourDomain.com (where 'your domain' is the domain name that you use). Virus attachments tend to have the following 'file extension': .exe, .pif. If you receive a .zip attachment and open it - make sure it doesn't contain a file with one of those extensions. Do not open attachments you haven't requested, even if they appear to be from people you know.

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