Top 12 Worst Computer Viruses in History

Ali Qamar Last updated: April 30, 2023 Read time: 20 minutes Disclosure

These 12 worst computer viruses have generated billions of dollars via stealth malicious activities.

With the advent of the first online virus in 1986, millions of computer viruses and worms infect computers daily, posing a severe threat to netizens’ online security. Such malware attacks are, so far, the most valuable tools of cybercriminals that can efficiently cause financial losses to the victim. This article lists the worst computer viruses in history that are still famous for their devastating damages.

Did you know the world’s first virus actually emerged more than three decades ago? Yes, apparently, the mother of all viruses, “Brain,” first surfaced online in 1986. Since then, many harmless or harmful malware have emerged and disappeared as cybersecurity people learned to cope with them. Nonetheless, a few of these are known for the worst computer virus attacks in the history of computer security.

This article tells you about the top 12 worst computer viruses to hit the digital arena. Furthermore, these viruses are all ranked according to the extent of financial damages they inflicted on the world. Hence, while they might not look so sophisticated, they undoubtedly proved to be the most successful viruses for cybercriminals.

Quick list of the worst computer viruses 

Here is a summary of the worst computer viruses in history

  1. Mydoom (38 billion): It caused an estimated $38 billion in damage and was responsible for a quarter of the world’s email traffic. Although it still exists, it’s behind only 1% of the world’s phishing emails.
  2. Sobig (30 billion): A worm in 2003 caused about 30 billion USD in damages and disrupted many businesses worldwide, including Air Canada.
  3. Klez (19.9 billion): It appeared in 2001, infected 7.2% of all PCs, sent fake emails, and evolved into more dangerous iterations over the years.
  4. Iloveyou (15 billion): It disguised itself as a plain text file in love letters and sent copies of itself to every contact in the infected computer’s email list.
  5. WannaCry (4 billion): It encrypted computer and cloud files, demanding payment for a key to unlock them, affecting 150 countries and 200,000 computers worldwide.
  6. Zeuz (3 billion): It was blamed for 44% of all banking malware attacks by Unisys in 2010 and comfortably infiltrated many organizations’ computers worldwide.
  7. Code Red (2.4 billion): It was a worm-caused DDoS attack against the White House website and infected almost one million hosts, leaving no trace in hardware and causing damages of 2.4 billion USD.
  8. Slammer (1.2 billion): It infected 200,000 computers and caused DDoS attacks on selected internet servers, mainly targeting banking computers in the US and Canada.
  9. CryptoLocker (665 million): Infected an estimated 250,000 systems, encrypted critical files, demanded a ransom, and cost victims an estimated USD 665 million.
  10. Sasser (500 million): Caused damages of around 500 million USD by crashing millions of computers worldwide, and he received a suspended sentence due to being a minor at the time.
  11. Melissa (80 million): It was a Word document that infected computers through a macro, then emailed itself to the top 50 contacts in the user’s email directory, causing economic damage of 80 million USD.
  12. Conficker (9 billion): it was a worm for Windows that exploited security gaps, infected nine million systems worldwide, caused nine billion USD in damages, and created a botnet by downloading another software on the affected computers.

The top 12 worst computer virus campaigns in digital history

Below we give you the costs, reach, key facts, and other details surrounding each virus. Nonetheless, this is by no means an extensive list of all digital viruses. Instead, they’re just the worst-known malicious programs known to exist so far.

Every day, we have about 127 million pieces of malware attacking digital denizens. So the list is infinite for any practical purposes. Our top twelve are the very worst but are not representative at all.

Not all the viruses listed below may fall into the category of “viruses” (technically). Instead, we have used the words “virus” and “worm” interchangeably here. This list merely intends to let you know the most devastating malware that have incurred huge financial damages until now.

1. Mydoom (38 billion)

The Mydoom outbreak is the worst virus attack ever to happen. Its estimated damage went as high as 38 billion USD (which would be 52.2 USD in current terms after adjusting for inflation). It also went by the name of “Novarg.” It was a worm that found its way around the internet mass emails. As this worm was active, it was responsible for about a quarter of the world’s email traffic.

As Novarg arrived into a system, it would scan it for fresh addresses. Then it sent copies of itself to those addresses. It also linked the infected computer to a botnet whose purpose was to carry out DDoS (Distributed Denial of Service) attacks. These attacks managed to shut down a website, or a server, by overwhelming it with junk traffic.

The funny thing about Mydoom is that it’s still around. But, unfortunately, it’s behind about 1% of the world’s phishing emails. If 1% strikes you as a meager fraction of those activities, think about this: the phishing traffic is currently about 3.4 billion emails daily. So that one percent represents thousands of millions of emails. So even 16 years after it was at the center of the world, Mydoom still has a life of its own, infecting those devices with the worse protection possible and still producing 1.2 billion copies of itself every year.

The Mydoom author was a wanted man. A quarter-million USD reward was available for his head, but nobody ever found him.

2. Sobig (30 billion)

Sobig appeared in 2003 as another worm, just like Mydoom. However, its success as the most dangerous cyber virus is second only to Mydoom’s as it managed to create about 30 billion USD in worldwide damage. It reached Europe, the US, and Asia. The authors released several Sobig versions quickly known from Sobig.A to Sobig.F. The last one was the worst.

This malware showed itself as a legitimate piece of software attached to emails.

It disrupted the activities of many businesses worldwide, with the Air Canada ticketing being the most famous problem during its time.