A computer virus is a _computer program_ 
(http://en.wikipedia.org/wiki/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, as  occurs in a _metamorphic virus_ 
(http://en.wikipedia.org/wiki/Metamorphic_code) . 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. Additionally, 
viruses can spread to other  computers by infecting files on a _network  file 
system_ (http://en.wikipedia.org/wiki/File_system#Network_file_systems)  or a 
file system that is accessed by another computer. Viruses  are sometimes 
confused with _computer worms_ (http://en.wikipedia.org/wiki/Computer_worm)  and 
_Trojan  horses_ (http://en.wikipedia.org/wiki/Trojan_Horse_(Computing)) . A 
worm, however, can spread itself to other computers without  needing to be 
transferred as part of a host. A Trojan horse is a form of malware  that presents 
itself as a legitimate program. 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_ (http://en.wikipedia.org/wiki/World_Wide_Web) , _e-mail_ 
(http://en.wikipedia.org/wiki/E-mail) , and _file  sharing_ 
(http://en.wikipedia.org/wiki/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_ 
(http://en.wikipedia.org/wiki/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_ 
(http://en.wikipedia.org/wiki/User_(computing)) . They  typically take up _computer memory_ 
(http://en.wikipedia.org/wiki/Computer_storage)  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_ (http://en.wikipedia.org/wiki/Data_loss) . 
There are many viruses operating in the general Internet today, and new ones  
are discovered every day.
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