Computer viruses are programs developed by mischievous or malicious computer programmers that are capable of performing a wide range of functions on 'infected' computers, from the benign to the malign. Malign viruses can corrupt or change data or programs, destroy computer files, or cause massive amounts of email to be generated, threatening the stability of networks by swamping them with data. Viruses are spread by transferring infected files from one computer to another. This can happen by transferring files on removable data disks, by accessing or downloading files on the internet or a network, or by files sent by email. Running these executable files or opening files containing infected macros can cause a computer virus program to run that can potentially do a great deal of damage.
Some viruses attack email programs.
By accessing a user's list of stored email addresses, a virus can self-replicate itself by sending copies of the virus to each email address. The multiplying effect of this strategy means that a virus can spread to a large number of computers all around the world in a short space of time. Computers hit by a virus attack can be severely damaged, and a lot of data can be lost or compromised. In the worst cases a computer's hard disk can be rendered useless, and all data on it is lost. The possibility of a virus attack is a very powerful incentive to conduct regular, thorough backups of programs and data.