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Top 5 Worst Computer Viruses

In this article I will be covering 5 of the worst computer viruses in history. The computer virus has been one of the most devastating influences on our computer systems. Viruses are malicious code designed to read, write and erase computer data. These malicious pieces of code are capable of causing a world of problems for your systems and yourself.

As the user, you are equipped with the ability to track down and erase all instances of the virus, or you could simply format everything and start over. However, both of these choices will lead to loss of time and data, potentially. These viruses are also capable of transferring the data on your computer to another, essentially violating your privacy. Fred Cohen was the first notable person to identify a computer virus, and defined it as a self-replicating program, designed to adversely modify our systems as a virus. Since them, the name virus has stuck.
There was a time when viruses were only capable of infecting a system through manual means, but thanks to the evolution of our technology, viruses today are mostly transferred through the internet.


One of the trickiest computer viruses was discovered for the first time in the year 2000, in the Philippines. It was essentially an email with the title ILOVEYOU which contained an email attachment (a virus). Around 10% of users online made the mistake of opening it. The attachment was a VB script. It worked by mailing the same email out to all the contacts in the users address book, while attacking the hard drive of the system it was on, this ultimately led to over $10 billion in damages.

2. Melissa
This virus was named after an exotic dancer from Florida, and it was created in 1999 by David Smith; it was a Microsoft Word macro. It worked by spreading itself through email. The virus would arrive in peoples Inbox with the subject title “here is the document you asked for, don’t show it to anybody else”. This was of course, designed to goad people into clicking on it, and for those who did, the virus would immediately forward itself to all the people in the users address book. After some time, the virus was able to replicate itself across a large portion of the web.

Though it didn’t adversely affect the internet, it did cause a big problem for many email services, as it increased traffic tenfold; this ultimately led to email services running considerably slower. David Smith was eventually arrested and sent to jail for 20 months, he was also fined $5000 by the US government. Though, it didn’t cause any real damage to any systems, it was one of the first viruses to gain any sort of recognition.

3. SQL Slammer
This virus successfully shut down South Korea’s online capacity for around 12 hours, affecting over 500,000 servers across the entire world.

This virus worked by affecting servers, not computers. Compared to other viruses, this one was very small, no larger than 380 bytes. It worked by generating random IP addresses and sending itself to these IP addresses. If the IP address belonged to a computer system that ran Microsoft’s SQL Server Desktop Engine (unpatched version), then that computer would become infected and send itself out to more random IP addresses. This virus was able to infect over 75,000 computers in less than 10 minutes. It created so much unwanted traffic that it overloaded thousands of network routers across the world, which eventually led to shut downs.

4. My Doom
My Doom was another virus, a worm to be exact, which arrived in 2004. It became another fast spreading email virus, much like ILOVEYOU. My Doom was able to infect two million computers, and created a massive denial of service attack that adversely affect the whole world wide web. It replicated by sending itself via email. It fooled people into clicking on it by using the subject line “Mail Transaction Failed”, and it would send itself out using the email address book of the user. The damages of this virus were said to be in the range of $38 billion.

5. The Klez Virus

With the natural progression and advancements in our technology, new viruses have been developed and pushed out there; one of such viruses, known to quite a number of people is the Klez Virus. It first arrived on the scene in 2001, and its unique characteristics lead to it floating around the internet for several months. The Klez Virus successfully replicated itself via email services. There were numerous versions of it; all made its behaviour either like a Trojan, worm or virus. As a virus it was capable of disabling the internet security software on an end users computer and act as a fake antivirus removal tool itself.

When the virus came out, there were many new versions that were released shortly after, these updated versions were capable of spoofing. It was also capable of deleting email messages and rendering a computer system inoperable.


Uchenna Ani-Okoye is a former IT Manager who now runs his own computer support website


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  2. Great share.
    I am so clear about these 5 computer viruses after reading your article.

    Iloveyou virus was first and most interesting. All are unconscious of virus like it.

    thanks to the author.