Deadly Viruses 101: The Worst Computer Viruses Of All Time

From opening an anonymous email to accidentally clicking on a website pop up, many of us have unfortunately had to deal with the nightmare that is a computer virus. While the consequences are varied — anything from shared information to a few hours of slow network traffic to even the infamous blue screen of death — computer viruses are something that many wish to avoid. Computer viruses have been an issue for decades and now contribute to billions of dollars of consumer losses each year. Let’s take a look at some of the most deadly viruses of all time — it may make you feel a bit better about that one time you unknowingly downloaded some virus-laden fake software.

Code Red and Code Red II

Attacking a vulnerability found in Windows 2000 and Windows NT, the Code Red viruses are actually credited with attacks on the White House. The vulnerable operating systems allowed for the virus to ultimately obtain access and control the machine. The individual behind Code Red was able to use infected computers for their own purposes, including ones involving crime. The computer worm ended up infecting nearly 360,000 machines — all on one day.

SQL Slammer/Sapphire

This web server virus made its mark in 2003 and eventually caused crashes of multiple important systems. Causing more than $1 billion in damages, the quick spreading virus caused more than just financial damage. Within 15 minutes following its first attack, the virus was documented to have infected almost half of the servers that directly support the functionality of the internet. Not only that, but all across the United States its damage was seen as ATM crashes, emergency service outages and flight cancellations.


This computer virus is reportedly named after an exotic dancer from Florida — this ‘Melissa’ has exclusively bad moves however. Released in the spring of 1999, the virus was spread through recipients opening a document, then the activated virus was automatically sent to 50 more people in that person’s email address book. When charged, the virus creator was accused of causing more than $80 million in damages to personal computers in both businesses and governments.


Hitting the internet in 2001, Nimda (admin spelled backwards) was a worm that quickly spread and actually was the fastest growing virus for years — only taking 22 minutes to be considered a serious threat. Targeting internet servers, the goal of the virus was to slow down internet traffic while creating a backdoor into the operating systems of its victims. Ultimately, thanks to Nimda many network systems crashed and one of its authors is still serving prison time.

The Klez Virus

The Klez virus made its debut in the early 2000s and survived for several months. Spread through email, the self replicating virus would attack anyone in the victim’s email address book. Hackers continued to evolve the virus and eventually it was able to spoof an email address — changing the “From” field to another user.


This virus, in the form of a self-replicating worm, shows no love when it comes to personal computers. Born in the Philippines, the virus began its global spread as an email supposedly containing a love letter from a secret admirer. Once the attachment was opened, the extension wreaked havoc on the system by making copies of itself, downloading password stealing applications, and emailed secret information to the hacker — all causing around $10 billion in damages.


By creating a backdoor in the operating system of the victim’s computer, MyDoom operated by creating triggers that allowed the virus to do denial of service attacks. Similar to other viruses, MyDoom also successfully spread by using victim’s email address books and eventually reached Google by creating search requests using the millions of corrupted computers. As the fastest spreading email worm, the virus not only spread quickly, but also had many important targets.

Sasser and Netsky

Surprisingly created by a 17 year old German student, this worm utilized a vulnerability in a Microsoft Windows operating system — but, unlike others, did not spread via email. Once the computer was infected, it searched for other vulnerable systems and began downloading the virus again. Spoofing emails, causing denial of service attacks, and spreading dangerous file attachments, Sasser and Netsky has been credited with accounting more than 25% of all computer viruses.


Causing nothing but mayhem, the past twenty years have seen some pretty destructive computer viruses. Hackers have used things like email and faulty operating systems to obtain access of a personal computer or network and have then sat back while the virus spreads like wildfire. Because of this it’s important to check different security reviews and programs to find ones that can handle against new and upcoming viruses. Causing trillions of dollars and damage (not to mention intense stress for the individual victims), it is no wonder that the world has seen deadly viruses like these — and we are sure to see more of them in the future.

Joanna Sommer

Joanna is the Senior Editor for InformedMag and is passionate about security and tech. She has been working in the home safety and security field for 5 years. Joanna loves to travel and enjoys going to hot yoga and Barre classes. She is dedicated to creating articles that both educate and help people make an informed purchasing decision.

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