Top 12 Worst Computer Viruses in History

Ali Qamar  - Cybersecurity Analyst
Last updated: October 11, 2023
Read time: 19 minutes

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 being the worst computer virus attacks in computer security history.

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 damage 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.