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 computer worms and
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, e-mail, and file sharing systems to spread, blurring the
line between viruses and worms. Furthermore, some sources use an alternative
terminology in which a virus is any form of self-replicating malware.
The term comes from the term virus in biology. 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 user. They typically take up
computer 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.
