No one knows just how many computer viruses are in circulation. New viruses are written every day, ranging from amateur or primarily benign to complex and extremely destructive. The latter type are less common, but over the years we have seen a few that not only caused a scared, but even impacted the way antivirus software companies engineer their products.
Brain
Although it is a subject of some debate, Brain (also known as ©Brain) is considered to be the first computer virus that was able to replicate itself from PC to PC. First written in January of 1986, it worked by infecting the boot sector of storage media that had been formatted with the DOS File Allocation Table (FAT) file system. Like many viruses that came after it, Brain was known by several aliases, including Pakistani, Lahore, Pakistani Brain, UIUC and Brain-A. It was also referred to as the "Pakastani Flu" by Businessweek magazine. The references to Pakistan were due to its country of origin and its creators. Basit and Amjaad Farooq Alvi, owners of Brain Computer Services were the ones responsible for the Brain virus. They claimed that they only wrote the virus to help them to better understand the extent of software piracy in Pakistan. In contrast to many of the malicious viruses that are written today, Brain was considered mainly harmless.
The Morris Worm
Often referred to as simply the "Internet Worm", the Morris Worm is considered to be the first computer worm to be distributed over the internet. Written by Cornell University student Robert Tappan Morris in 1988, the worm caused significant damage and resulted in the very first conviction under the 1986 Computer Fraud and Abuse Act. Tappan released the worm from MIT, in hopes of disguising the fact that it had originally come from Cornell University. Due to Morris' design of the worm, instead of simply infecting computers, it attempted to re-infect many machines that were already infected (and often succeeded). This caused the spread of the worm to get drastically out of hand and cause significant damage. As fate would have it, Morris currently holds a position as an associate professor at MIT.
Melissa
Created by David L. Smith, the Melissa virus was a powerful macro mailer that infiltrated countless email boxes and tricked people into opening it, which subsequently caused the virus to be sent to the first 50 people in their address books. Smith maintains that he did not intend to cause any harm when he wrote the virus, but rather that it was a simple experiment to determine whether or not it would work.
Love Bug
Also known as the "ILOVEYOU" worm, the Love Bug was unleashed in mid 2000. Functioning similar to Melissa, the Love Bug disguised itself as a love letter and tricked it's recipients into running the attached file. Instead of sending itself to the first 50 addresses in the host's address book, it sent itself to everyone.
Code Red
2001 brought one of the first "network worms". Code Red exploited a flaw in Microsoft's software and spread extremely rapidly since it only required a network connection to travel (not a human opening or executing an attachment). Although Microsoft had released a patch a month prior to the release of Code Red that would have remedied the vulnerability, many affected users did not utilize the patch in time.
Sasser
Similar to Code Red, in 2004 Sasser took advantage of a Microsoft flaw and caused many computers to crash and reboot in perpetuity, an effect that many believe was not intended by the writers of the virus but was rather a product of poor programming.
Although there have not been any real newsworthy viruses announced in the past few years, it certainly does not mean that they don't exist. Virus writers today make every effort to remain below the radars of the antivirus companies, as the lifespan and potency of their products depend on remaining undetected. With the sophistication of today's antivirus applications, it is far more difficult for amateur virus writers to cause any significant damage.
