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Included in many viruses:
•-Trojan horses - A Trojan horse is just a computer program. The program pretends to do one thing (like claim to be a picture) but actually does damage when you start it (it can completely erase your files). Trojan horses cannot replicate automatically. Computer viruses are called viruses because they share some traits of types of biological viruses.
•-Worms - A worm is a piece of software that uses computer networks and security flaws to create copies of itself. A copy of the worm will scan the network for any other machine that has a specific security flaw. It replicates itself to the new machine using the security flaw, and then starts replicating.
•-E-mail viruses - An e-mail virus will use an e-mail message as a mode of transport, and usually will copy itself by automatically mailing itself to hundreds of people in the victim's address book.
In computer security, a computer virus is a self-replicating computer program that spreads by inserting copies of itself into other executable code or documents.
•A computer virus behaves in a way similar to a biological virus, which spreads by inserting itself into living cells.
•Extending the analogy, the insertion of a virus into the program is termed as an infection, and the infected file, or executable code that is not part of a file, is called a host.
•Viruses are one of the several types of malicious software or malware.
A computer virus will pass from one computer to another like a real life biological virus passes from person to person. For example, it is estimated by experts that the Mydoom worm infected a quarter-million computers in a single day in January of 2004. In March of 1999, the Melissa virus spread so rapidly that it forced Microsoft and a number of other very large companies to turn off their e-mail systems until the virus could be dealt with. Another example is the ILOVEYOU virus, which occurred in 2000, had a similarly disastrous effect.
In common parlance, the term virus is often extended to refer to worms, Trojan horses, and other sorts of malware; viruses in the narrow sense of the word are less common than they used to be, compared to other forms of malware.
While viruses can be intentionally destructive, for example, by destroying data, many other viruses are benign or merely annoying. Some viruses have a delayed payload, which is sometimes called a bomb.
For example, a virus might display a message on a specific day or wait until it has infected a certain number of hosts. The predominant negative effect of viruses is their uncontrolled self-reproduction, which wastes or overwhelms computer resources.
Today, viruses are somewhat less common than network-borne worms, due to the popularity of the Internet. Anti-virus software, originally designed to protect computers from viruses, has in turn expanded to cover worms and other threats such as spyware, identity theft, and adware.