The 6 Worst Computer Viruses of All Time

Since Halloween is coming up, there has been lots of talk of scary things - ghosts, witches, zombies, and the like. But really, one of the scariest things out there are the viruses that attack our computers. Since cyber criminals have invented these nasty bugs, billions of dollars have been lost and countless problems created. Here are the very worst, most insidious viruses that have popped up to terrorize us.

1. Melissa

One of the very first viruses to capture America’s attention was Melissa. In 1999, a man named David L. Smith created a virus that spread through email messages. He claimed he named the virus after an exotic dancer in Florida (you know, like you do). The virus tricked people into opening emails with subject lines like, “Here’s the document you asked for.” As soon as the virus was activated, it would automatically send itself to the top 50 people in the recipient’s address book - instantly creating a deadly chain.

Ultimately, the FBI got involved after government networks were affected. A lengthy trial ensued, and Smith received a 20-month jail sentence and a $5,000 fine. He was also prohibited from accessing computer networks without permission from the court. Although the virus didn’t completely incapacitate the Internet (like future menaces would), it did get the general public to start talking about computer viruses.


Usually, ‘I love you’ are three words we long to hear. However, the ILOVEYOU virus ended up being a true nightmare, insinuating its way into more than 500,000 PCs. The worm virus originated in the Philippines and was transported through email. The message stated that it was a love letter from a secret admirer with an attachment named LOVE-LETTER-FOR-YOU.TXT.vbs (vbs from the Visual Basic Scripting program the hacker used to create the virus). As soon as you clicked on the attachment, the virus copied itself and hid the copies in several different folders on your hard drive. ILOVEYOU also downloaded a file called WIN-BUGSFIX.EXE on your computer, which ended up stealing your passwords.

No one has directly taken credit for starting the virus, although some think it was a man named Onel de Guzman from the Philippines. Overall, the ILOVEYOU virus caused more than $15 billion in damages!  

3. Code Red

The Code Red and Code Red II viruses arrived in 2001. These worms capitalized on an operating system vulnerability with Windows 2000 and Windows NT. The first Code Red was a worm that started a ‘distributed denial of service,’ or DDoS, attack on the White House - every computer infected with Code Red tried to contact the Internet servers at the White House at the same time, causing a system overload.

Code Red II worked to override a computer’s operating system, giving complete control to the hacker. This virus was particularly dangerous since computers could be accessed remotely to commit crimes (without the owner’s knowledge). Microsoft eventually released software patches, so the worms would no longer be effective. However, by then, there were nearly $2.6 billion in damages and over 1 million PCs had been affected.

4. Slammer

2003 saw a new Web server virus hit the Internet called the SQL Slammer, also known as Sapphire. Within minutes of arriving, the Slammer virus doubled its victims every few seconds. Nearly half of all servers were affected within 15 minutes! In fact, Bank of America’s ATMS went down, the city of Seattle experienced 911 service outages, and Continental Airlines suffered from ticketing errors, resulting in tons of cancelled flights.

More than 200,000 PCs were affected and over $1.2 billion in damages were reported. The Slammer virus taught consumers that hackers would use any type of virus they could to exploit and work their way around antivirus software.

5. MyDoom

Another worm virus arrived in 2004 called the MyDoom, or Noravg, virus. This one was similar to the Code Red virus in that backdoors were created on a computer’s operating system. Another outbreak occurred later that year when search engine operators were affected. Companies like Google ended up getting millions of search requests from computers that had been corrupted by the virus. Some sites even ended up crashing because of MyDoom.

It’s believed that MyDoom affected one in every 12 email messages at the time, with harm to more than 2 million computers. It’s also one of the costliest viruses to date with over $36 billion in damages!

6. Conficker

A more recent viral outbreak occurred in 2009 with worm Conficker (that also went by the names Downup, Domnadup, and Kido). The virus affected as many as 15 million Microsoft Windows computers (more than any other virus up until that time). It was spread through removable drives like hard disks, smartphones, and flash drives by adding corrupt files to each new device. Conficker also worked to steal financial data and other personal information from users’ systems. The virus most likely caused around $9 billion in damages, and was extremely far reaching, affecting computers in more than 200 countries around the globe.

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