An email virus is a virus that is distributed through the use of emails, most frequently as an attachment to an email.
When the unsuspecting recipient opens the attachment to the email, the virus is released onto his or her computer. Most email viruses consist of a macro virus that enters the address book on your computer and then spreads itself to all of your contacts.
The virus then automatically sends emails from your computer to all of the contacts in your address book, such as your family, your friends, and your associates. Anyone receiving "your" email, who opens the attachment of the infected mail, automatically has them same cycle occur in his computer. Such a virus can thus spread rapidly to many computers, in a matter of hours.