Computer VirusA computer virus is a computer program that can copy itself and infect a computer without permission or knowledge of the The original may modify the copies or the copies may modify themselves， as occurs in a metamorphic A virus can only spread from one computer to another when its host is taken to the uninfected computer， for instance by a user sending it over a network or carrying it on a removable medium such as a floppy disk， CD， or USB Additionally， viruses can spread to other computers by infecting files on a network file system or a file system that is accessed by another Viruses are sometimes confused with computer worms and Trojan A worm， however， can spread itself to other computers without needing to be transferred as part of a A Trojan horse is a file that appears harmless until In contrast to viruses， Trojan horses do not insert their code into other computer Many personal computers are now connected to the Internet and to local-area networks， facilitating their Today's viruses may also take advantage of network services such as the World Wide Web， e-mail， and file sharing systems to spread， blurring the line between viruses and Furthermore， some sources use an alternative terminology in which a virus is any form of self-replicating The term comes from the term virus in A computer virus reproduces by making (possibly modified) copies of itself in the computer's memory， storage， or over a This is similar to the way a biological virus Some viruses are programmed to damage the computer by damaging programs， deleting files， or reformatting the hard Others are not designed to do any damage， but simply replicate themselves and perhaps make their presence known by presenting text， video， or audio Even these benign viruses can create problems for the computer They typically take up computer memory used by legitimate As a result， they often cause erratic behavior and can result in system In addition， many viruses are bug-ridden， and these bugs may lead to system crashes and data There are many viruses operating in the general Internet today， and new ones are discovered every
