A computer virus is a computer program that can replicate itself and spread from one computer to another. The term "virus" is also commonly but erroneously used to refer to other types of malware, including but not limited to adware and spyware programs that do not have the reproductive ability. A true virus can spread from one computer to another (in some form of executable code) when its host is taken to the target computer; for instance because a user sent it over a network or the Internet, or carried it on a removable medium such as a floppy disk, CD, DVD, or USB drive.
Viruses can increase their chances of spreading to other computers by infecting files on a network file system or a file system that is accessed by another computer.
HOW DOES COMPUTER VIRUS AFFECT YOUR COMPUTER
Though they're not a living thing like you and me, computers can get "sick" from viruses, too.
A computer virus is a software program designed to replicate itself and spread to other machines. In most cases, the program is "malicious," meaning its purpose is to cause the computer malfunction in some way
In general usage, the term "computer virus" includes all forms of "malware," or malicious software.
KINDS OF COMPUTER VIRUSES
Famous computer viruses
1. ILOVEYOU - (2000) One of the most widespread and rapidly spreading viruses ever, the ILOVEYOU virus spread via e-mail, posing as an executable attachment sent by a friend from the target's contact list.
2. Code Red - (2001) IIS on Windows servers were the target of this virus. It also launched denial of service (DoS) attacks.
3. Nimda - (2001) Nimda used seemingly every possible method to spread, and was very effective at doing so. Nimda is notable for being one of the fastest spreading and most widespread viruses ever.
4. Melissa - (1999) The Melissa virus is notable because it is a Word macro virus. It cleverly spread via e-mails sent to contacts from the infected users' address books.