"A virus is a type of program that can replicate itself by making (possibly modified) copies of itself. The main criterium for classifying a piece of executable code as a virus is that it spreads itself by means of 'hosts'.
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 disk. 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 worms. A worm, however, can spread itself to other computers without needing to be transferred as part of a host. 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, e-mail, and file sharing systems to spread, blurring the line between viruses and worms."