Top 10 Worst PC Viruses

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Top 10 Worst PC Viruses

Kevin R. Smith


I came across an interesting find today from some of our peers at They make a list--and a good one at that--of the Top 10 Worst Computer Viruses of all time.

Internet security tends to have a short memory once we're past a computer virus outbreak. We tend to breath a collective sigh of relief whether we dodged the problem altogether or finally got a virus cleaned up.

After that, we move on to other tasks and before we know it (especially if we weren't hit by a particular virus ourselves), we've forgotten how damaging a particular virus or worm was.

As I read through this list, I found myself nodding again and again and remembering some what dread these malware nasties caused for so many businesses and personal computer users.

 The things that stood out for me as I perused the list were:

  1. There are three types of viruses:
    1. Those that cause damage intentionally.
    2. Those that cause damage unintentionally, i.e. mostly collateral damage/clean-up damage for those who have to take care of the mess
    3. Those that don't do much of anything and are really just irritating.
  2. Even years later some viruses/worms/malware are still causing problems.
  3. As viruses have grown in sophistication, an ounce of prevention is still worth a pound of cure.

    Put another way: look at the reviews we've put together at our homepage,, choose the software that's right for you, and you're much less likely to have issues.





Virus Payload / Damage

1 MyDoom Automatically used your address book to send itself to... everyone in your address book. Wash, rinse, repeat for everyone in those people's address books and so on.

What was particularly vexing though was that it also used the Kazaa file sharing network, leveraging the peer-to-peer network to spread even further.

From there the entire network then turned itself against SCO networks at, launching a massive distributed-denial-of-service (DDoS) network against the company's website.

Whether you like SCO or not, such DDoS attacks put chills into many a network administrator, who feared their networks could just as easily fall victim to such attacks.
2 Nimda According to pcAuthority, Nimda went "from nowhere to become the most common virus online in 22 minutes...."

Nimda used a variety of methods to target users, including: email, network shares, Microsoft IIS vulnerabilities, and even other web sites to spread from one computer to the next.

Once there, it continued to attack other computers in similar fashion.

The real threat--and damage--from Nimda was the resources it could quickly consume on a particular PC--and a network--as the worm hopped and spread from one machine to the next, overloading network switches and mail servers in the process.
3 Melissa The backstory behind this worm is hard to believe but true: a New Jersey hacker wrote the virus to impress a stripper he met in Florida. No, really.
"The real damage of Melissa was not in the code itself, but in its spamming capabilities.

The software caused massive overload of email systems and generated enough traffic to make it highly visible.

Current computer malware writers have taken note of code like Melissa and now fly much lower under the wire to attract less attention.
4 Storm The goal of Storm was to create a botent. It succeeded. And as the article points out,
"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."
Storm's real threat came in how cleverly it tricked people into clicking on it by making reference to current events and holidays.

Somehow the use of these in the various videos files and greeting cards it disguised itself as, put people at ease and helped the botnet quickly spread and grow.
5 ExplorerZip Like many worms / viruses, ExplorerZip was / is spread via email.

This virus, unlike some of the others which only setup botnets for spamming or only spread for the purposes of spreading, actually did real damage to your files if you got infected, writing nothing but zeros to your Microsoft Word docs and even doing what pcAuthority describes as, "some damage to the operating system itself."

6 Conficker We've covered both the "What is Conficker?" and the "How to Remove Conficker" quite a bit on our blog, and while so far it's, "just another botnet builder just like most other malware," according to pcAuthority's Iain Thomson, one fact remains:

millions of computers were infected, taken over really, and because of that, the botnet can be used for just about anything.
It's not just a simple virus or worm as it relies on very sophisticated components like the new MD6 hashing standard.

I'm sure we haven't heard the last of this bugger.
7 Klez Klez first showed up on the scene some seven years ago caused many a user and network administrator a *lot* of grief. Once again quoting
"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."

All-in-all it really was a nuisance virus rather than something truly destructive, though email systems administrators who had to clean up the mess it left in its wake likely have a different opinion.
8 Elk Cloner
"It had little in the way of a payload. Every fiftieth 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."
The thing the article points out that I hadn't really considered was that it spread though "boot sector" infection.

On the surface this might not seem like much, but it really helped make the whole boot sector infection a pretty standard technique in the process of spreading viruses.
9 Brain Known for being the first Microsoft DOS virus, it was originally developed to help two programmer brothers stop piracy of their medical software.

The problem came because the snippet of anti-piracy code was yanked out by other unscrupulous coders who then turned it into its own virus.
10 Creeper Other than being widely known as the first computer virus, there really wasn't much to it.
It displayed a message, "I'm the creeper, catch me if you can!" and even came with its own removal program, "Reaper."
Editor's note: Rankings were re-numbered from the original list to make the concept of a "Top 10" end at "#1" instead of "#3" as the original source had them.


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