You've Got Email Virus

Viruses have been around for sometime in the world of computing.  They have become much more prevalent in today's society thanks to modern technologies such as the internet.  Malicious code writers essentially changed the environment of computing with the creation of email viruses.  Take Melissa for example.  Released in 1999, this infection is known as one of the most devastating viruses of all time.  Melissa distributed itself through Microsoft Word documents distributed via email.  Here is how it functioned: 

The Email Virus Origin

The virus was originally created as a Word document and was then uploaded via email to an internet newsgroup.  Any recipient who opened the email, downloaded the document and opened it on their computer, unknowingly triggered Melissa's payload.  From there, the virus sent itself as a document to the first 50 contacts in the victim's address book.  The email was attached with a friendly note which included the recipient's name.  This was done to make the virus appear harmless and trick them into opening it.  It then created 50 new infected documents from that victim's machine.  At this continuous rate, Melissa quickly became the fastest spreading virus seen by anyone at the time.  The virus was so severe that it resulted in a number of large commercial companies disabling their email systems.  

Melissa was so powerful because it capitalized on a vulnerability found in the Microsoft Word programming language known as VBA (Visual Basic for Applications).  VBA is a complete language that can be programmed to perform actions such as modifying files and distributing emails.  It also includes a rather useful yet dangerous function known as "auto-execute".  The Melissa virus was programmed by inserting malicious code into a document, enabling it to be executed whenever someone opened it.  

The ILOVEYOU virus, which was first detected in May of 2000, was much more simple than Melissa.  The malicious code it contained came in the form of an attachment.  Any recipient who clicked on the attachment unknowingly executed the code.  This email virus then distributed copies of itself to contacts in the user's address book, enabling the infection to spread at a rapid rate.  Because ILOVEYOU was also known to unload different types of infections, some experts have labeled it a Trojan rather than a virus. 

Fueling Email Viruses

Since they are known to exploit common vulnerabilities in word processing applications, email viruses fall under the classification of macro viruses.  Because of their wide spread nature, most Microsoft applications are equipped with a feature known as Macro Virus Protection, which helps to prevent ths type of infection.  When this feature is enabled, a dialog box is displayed to warn the user of any document attempting to execute a malicious code.  Unfortunately, many users have limited knowledge on macros or macro viruses, causing them to ignore the warning and unknowingly allow the infection to launch. 

This type of feature would be useless against the ILOVE virus which was entirely human powered.  Overall, the infection was fueled by the willingness of a human recipient to click on the virus and initiate it's execution.