What are the different types of computer viruses?

There are a few different types of computer viruses.  The typical virus is the virus explained in the previous section.  There are also email viruses, worms, and Trojan horses.

Email viruses first started showing up in the 1990s when the use of email became widespread.  Email viruses are able to spread more quickly than traditional viruses, because they are sent via email.  One of the most impressive viruses to hit computers was the Melissa virus, which happened in March 1999.  The virus was attached to a word document that was downloadable from an Internet newsgroup.  Anyone who downloaded that document executed the virus.  The virus was able to then make its “attack” by sneaking into the person’s email account and sending the infected document to the first fifty people in the address book, with a friendly subject line.  These people would see the email message, and click on it to open it up.  This would then execute the virus, and then the document would be sent to the first fifty people in the new address book.  This became the fastest spreading virus ever seen.  The damage was quite significant.  The virus infected a central file called NORMAL.DOT.  Any file saved later would also contain the virus.
Another notable email virus was called the ILOVEYOU virus, which infected computers on May 4, 2000.  An email was sent out with a virus infected attachment.  The recipient double-clicked on the attachment, allowing the virus to execute—automatically sending the attachment to the recipient’s entire address book.  The effects were disastrous as well—destroying files and programs on the recipient’s computer.

Worms are pieces of code that use computer networks and security holes to replicate themselves and wreak havoc on systems.  Worms are computer programs that are able to copy themselves from machine to machine, through computer networks.  In July 2001, the Code Red Worm copied itself over 250,000 times in a nine hour span.  It specifically attacked servers of Windows NT 4.0 and Windows 2000 running Microsoft Internet Information Server.  The worm searched for unsecured servers, and copied itself to them.  The Code Red Worm was made to have three major effects:
• Replicate itself for the first twenty days of infection
• Replace the victim’s web pages with a page stating, “Hacked by Chinese.”
• Orchestrated an attack on the White House web server in an attempt to overwhelm it.

Trojan Horses are actually normal computer programs.   A Trojan Horse pretends to be a regular downloadable program, such as a game, but actually does something different—most likely does damage to a computer, like erasing a disk.  This is different from a regular virus, because it doesn’t attempt to reproduce itself.  Trojan Horses do not affect a great number of people because they are discovered quickly.

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