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Computer viruses can be a nightmare. Some can wipe out the information on a hard drive, tie up traffic on a computer network for hours, turn an innocent machine into a zombie and replicate and send themselves to other computers. If you've never had a machine fall victim to a computer virus, you may wonder what the fuss is about.
Am going to take a look at some of the top 10  worst computer viruses to cripple a computer system in history . Let's start with the love virus.

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1. I Love You

They say you always hurt the ones you love. In 2000, this was taken to extremes when the ILoveYou virus caused $5.5bn in damages.
The concept was pretty simple: a user receives a file from a known email contact under the title 'LoveLetter' or 'ILoveYou'. When the attachment is opened, the virus is launched. After infecting the host, the virus then took control of the user's email program and sent the same 'ILoveYou' message to every user in the host's address book.
Love must have been in the air, because the virus was potent enough to infect some 10 per cent of internet-connected machines at its peak. At a time when many users were still trying to learn the finer points of the internet, ILoveYou was a major wakeup call to some of the dangers on the web.
Everybody wants to be loved and ILoveYou was brilliant social engineering. It helped that the virus was spammed out in the early days of internet use and there were a lot of newbies online who had only a vague idea about viruses and how dangerous they could be.
Email was a trusted format and, because the messages came from people the recipient actually knew, the likelihood of them being opened was much higher.
Things are different today, although there are still plenty of people who get caught by social engineering attacks, but ILoveYou makes it so high in the list because it was a brilliant piece of social engineering.

2. Sasser
Just how much damage can a virus do? Well, take the Sasser worm as one example. This relatively simple little attack managed to cripple airlines, news agencies and even knocked out government systems.
Perhaps most frustrating, however, was that Sasser infection was very easy to prevent. The vulnerability which the attack exploited had been patched for months, and all users had to do was install the most recent security updates from Microsoft.
Sasser was a stark warning that has yet to be heard by many. Unpatched systems are still pervasive around the world, leaving users vulnerable to Sasser and countless other malware attacks that target patched vulnerabilities.

3. MyDoom
Ah yes, the old 'infect the host then resend to the entire address book' attack method. Like many other attacks, MyDoom used the tried-and-true practice of spreading through email and address books.
But MyDoom went a step further and targeted peer-to-peer networks. The worm not only spread itself through address books but through the shared folder of users who ran the Kazaa file sharing application.
While definitely skilled programmers, MyDoom's creators also seemed to be fans of good old-fashioned vigilante justice. One of the early tasks performed by infected users was to take part in a denial-of-service attack against SCO, the infamous software vendor that once tried to lay claim to the patents for Linux.

4. Nimda
A week after the 11 September atrocities a new virus hit the internet in a big way. Nimda was one of the fastest propagating viruses in history, going from nowhere to become the most common virus online in 22 minutes, according to some reports.
The reason for this speed was that Nimda used every trick in the book to spread itself. It used email, open network shares, IIS vulnerabilities and even web sites to spread. It hit pretty much every version of Windows available and appeared all over the place.
In the paranoid days after the terrorist attack some speculated that this was a digital 11 September, and some security consultants got large speaking fees for suggesting just that. In fact, it was nothing of the sort and was just another attempt at large scale infection.

5.  Melissa

Melissa was created by David L. Smith in 1999 and is based on a Microsoft Word macro. He intended to spread the virus through e-mail messages. The virus prompts the recipient to open a document and by doing so the virus gets activated. The activated virus replicates itself and will be transferred to 50 persons whose address is present in the recipient’s e-mail address book. The virus was spread rapidly after it was unleashed by Smith. The increase in e-mail traffic due to the virus forced some companies to block e-mail programs until the virus attack was controlled.

6. Storm
Before Conficker came around and got everyone worked into a lather, Storm was the big bad botnet on the block. First appearing in early 2007 as a fake news video on European flooding, the Storm malware menaced users for more than a year.
The huge botnet was also influential for its continued use of social engineering tactics. The malware disguised itself as everything from video files to greeting cards, and attacks were continuously refreshed to coincide with holidays and current news events.
While Storm has since been eclipsed by newer botnets, the name still brings to mind one of the most menacing attacks seen in recent years.

7. Conficker
The global catastrophe that wasn't, the third form of the Conficker attack provided nice theatrics but little in the way of actual damage.
The premise was pretty simple: Conficker.C would spread to as many machines as possible throughout March. Each infected machine was given a huge list of domains, one of which would be contacted by 1 April.
The deadline made all the difference. Now, Conficker wasn't just a simple malware infection, it was a 'ticking time bomb', and a looming menace that would unleash carnage. Or at least that's what the story turned into when unscrupulous security vendors and tech-newbie news outlets got hold of the story.

8. SQL Slammer/Sapphire SQL
Slammer/Sapphire virus caused a damage of more than $1 billion and the affected networks included Bank of America’s ATM service, Continental Airlines etc . A few minutes after the infection of the first Internet server, the number of victims of the Slammer virus doubled every few seconds. After Fifteen minutes of the first attack, half of the servers that act as the pillars of the Internet were affected by the virus.
The Slammer virus taught a valuable lesson: It's not enough to make sure you have the latest patches and antivirus software. Hackers will always look for a way to exploit any weakness, particularly if the vulnerability isn't widely known

9. Klez
Klez is a persistent little devil, and variants are still doing the rounds today, seven years after it first turned up.
The most common varient, Klez H, spoofs email addresses by randomly picking one from an infected machine before sending itself on to other users. This makes backtracing the identity of the infected machine particularly difficult, since any email stored for any reason can be used.

It exploits a vulnerability in Outlook that allows it to boot up automatically on unpatched systems. It's a cunning little devil but for all its ingenuity I still want to strangle the writer.

10. Elk Cloner
Elk Cloner was written by a 15-year old high school student called Rich Skrenta as a practical joke. Unfortunately for him the joke turned bad very quickly.
The virus was developed for the Apple II system and was a boot sector virus that spread via floppy discs. Apparently Skrenta was a fan of pirated games and would swap them with his friends, sometimes with little messages added. After one too many of these infected discs, he devised a way to alter discs automatically and the Elk Cloner virus was invented.
It had little in the way of a payload. Every 50th time a person booted an infected disc the software ran a little program on the computer screen, and that was it. Nevertheless it was a serious annoyance and was a harbinger of things to come.

By Joe