A computer virus is an illegal and malicious program or code designed by people to infect other software by attaching itself to any software it contacts. The term computer virus is related to a biological virus. The word virus is a Latin word, which means poison. Similarly to biological virus, computer virus can spread if it is not removed fast. Virus code will copy itself into one or more large programs when it is activated. When these infected programs are run, the virus is executed and spreads further. Computer virus infects only executable programs and not pure data. However, some data, such as text files, spreadsheets can be infected also if they are used by infected application programs. Computer viruses often infect many programs on the same system or even parts of the operating system in an attempt to spread themselves as far as possible. Also, 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. Virus can be very malicious and can actually destroy a computer. It can slow a computer down, mess up or change the data in files and programs, mess up the boot sector, give access to the computer to other users, and physically damage the hard drive or other parts of the machine. Because of the speed at which computer works, a virus can make a lot of copies of itself in seconds. If it gets into a network it can spread to thousands or millions of users within weeks or even days. Computer viruses have become a serious problem in the resent years. Currently, there are more than 62,000 known computer viruses in the world and more are being discovered every day.
