An e-mail virus is computer code sent to you as an e-mail note attachment which, if activated, will cause some unexpected and usually harmful effect, such as destroying certain files on your hard disk and causing the attachment to be remailed to everyone in your address book. Although not the only kind of computer virus, e-mail viruses are the best known and undoubtedly cause the greatest loss of time and money overall. The best two defenses against e-mail viruses for the individual user are:
(1) a policy of never opening (for example, double-clicking on) an e-mail attachment unless you know who sent it and what the attachment contains, and
(2) installing and using anti-virus software to scan any attachment before you open it.
Business firewall servers also attempt, but not always successfully, to filter out e-mail that may carry a virus attachment.