A Computer Virus is a computer program that can copy itself and infect a computer without permission or knowledge of the user. The original may modify the copies or the copies may modify themselves.. 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, USB drive or by the Internet. Additionally, viruses can spread to other computers by infecting files on a network file system or a file system that is accessed by another computer. Viruses are sometimes confused with Trojan horses. A worm, however, can spread itself to other computers without needing to be 
transferred as part of a host. A Trojan horse is a file that appears harmless until executed. In contrast to viruses, Trojan horses do not insert their code into other computer files. Many personal computers are now connected to the Internet and to local-area networks, facilitating their spread. Today's viruses may also take advantage of network services such as the World Wide Web, email, and file sharing systems to spread, blurring the line between viruses and worms. A computer virus reproduces by making (possibly modified) copies of itself in the computer's memory, storage, or over a network. This is similar to the way a biological virus works.
Some viruses are programmed to damage the computer by damaging programs, deleting files, or reformatting the hard disk. Others are not designed to do any damage, but simply replicate themselves and perhaps make their presence known by presenting text, video, or audio messages. Even these benign viruses can create problems for the computer. They typically take up memory used by legitimate programs. As a result, they often cause erratic behavior and can result in system crashes. In addition, many viruses are bug-ridden, and these bugs may lead to system crashes and data loss.
There are many viruses operating in the general Internet today, and new ones are created and discovered every day.
