There are two things that bring fear to any computer user or any computer network administrator, one is a hacker and the other is a virus. A hacker is a person who breaks into your computer in order to cause harm. A virus is a little different, it's a software that has been written by a person which if executed could cause harm to your computer.
There are many ways in which a virus can creep into your computer. The most powerful way is to get in through the internet. Or the virus could be embedded in a CD, DVD, diskette or a flash-drive. In the cyber world, a virus could cause so much commotion and fear that the news will travel faster than the actual virus itself.
A virus is a program written by a person, to intentionally cause harm to another computer. Viruses attach themselves to other programs, boot sectors of your disk and replicate whenever these programs are executed. There are many types of viruses. Some are written for fun and does not cause much trouble. Some viruses, on the other hand, can cause catastrophic damage to your system by deleting files, deleting system files, formatting your hard disk, disrupting the boot sequence and so on. Just because your computer is infected by a virus does not mean that your computer is totally destroyed. A computer might be infected with a virus, but the destructive code or the order to harm a computer will only be executed if certain criteria are met. The code is executed if say, a certain file is opened or on a certain day and so on.
Then there are viruses which are called Trojans and Worms. A worm is a virus which gets into your computer and does no harm in the beginning. The virus evolves within the computer, slowly destroying your computer. A Trojan has the same technique as the Trojan horse created by the Romans to bring down Troy. The virus is concealed so it's not easy to identify it. Once the virus is in and is executed, it is free to cause chaos within the computer. Most Worms get into the computer by the help of a Trojan horse.
For these reasons there are companies which are dedicated to creating anti-virus software. These anti-virus guards, as they are called, help protect your computer from virus attacks. When a computer is protected by a virus guard, when the virus tries to enter or execute inside your computer, the virus guard will detect it and warn the user. Then the user can either choose to quarantine the virus or delete it and the files which have been affected. There are many viruses created everyday, so a virus guard needs to be up to date. A user may download the relevant updates from the vendor's respected website. The toughest types of viruses to detect and destroy are Trojan horses and worm attacks. A worm will be especially difficult to track down and destroy, particularly if it has spread throughout your computer. Never the less, anti-virus guards have proven to be very effective against the thudding attack of viruses. But the fight against the viruses is similar to the fights against good and evil and will see no end for many more years to come.
Summery: This article explains what computer viruses are, why they are created, how they cause damage, different types of viruses found today and how they can be removed effectively through anti-virus software.