Computer Viruses
A computer virus is software that can copy itself and infect a computer without the permission or knowledge of the owner. A computer virus must piggyback on top of some other program or document in order to launch. Once it is running, it can infect other programs or documents.
An original computer virus can only spread from one computer to another when its host is taken to the target computer. For example, by a user sending it over a network or the Internet, or carried it on a removable medium such as a floppy disk, CD, DVD or USB drive.
Initially, worms and trojan horses are diffrent. They exploit the security vulnerabilities of a network or PC without the need to transfer part of the host. In the age of the Internet the line between original computer viruses and new forms of malware like worms and trojan horses becomes a bit vague. Nowadays, viruses can also use the Internet, IM programs, P2P networks etc. in order to infect PCs and networks, which make them similar to worms.
Computer viruses may cause harm to the hosted data of a computer system,
functional
performance or networking throughput when they are executed.