A computer virus is a program or piece of code designed to hide in the background and to perform one or more of the following activities
· Replicate itself
· Attach to other programs or instructions
· Perform unintended actions
In order to be called a “Virus” a program needs not to perform outright damage. Some virus programs do not cause any damage to the computer. One of the most costly types of damage that a virus can cause is the modification of the accuracy of data.
Another embarrassing effect of some viruses is the disclosure of confidential documents, passwords, PIN numbers etc… to Internet websites and e-mail to external parties. Until all instances of an affected virus are deleted, the virus maintains the capability of self-replication and the process may start all over again.
A virus needs a place to live, "host" - the particular hardware and software environment on which it can run. When an infected program is executed, the embedded virus is executed too, thus propagating the "infection" without the knowledge of the user. Computer viruses are designed to attach to many files or disks without being detected and to cause damage to data held on the affected computer.
Some viruses also slow down a users system, disable certain functions, or cause erratic system behavior. Computer viruses often infect many programs on the same system or even parts of the operating system. Users can unknowingly transmit a virus from one system to another by copying infected program files from one machine to another or by using an infected program downloaded from the Internet.
The first common computer virus, called “Brain” virus, was written in 1986 by two brothers in Pakistan. The majority of viruses are written by hackers. Some of the known viruses and their effects are given below.