The worst computer viruses of all time

Although many believe that the worst is yet to come, we have already seen some pretty savage computer viruses that have caused damages in the billions and left a lasting impact on the tech industry. In the hope that we can learn from what has gone before, below are some of the worst computer viruses of all time.


Unbelievably this virus was created by a 17-year-old German student who released it as a birthday present to himself when he turned 18 in 2004. It is estimated to have infected around a million PCs and caused around $500 million of damage by its unique method of dissemination – using each infected machine to find vulnerable machines on its network and spreading instantly to them too.


When the Melissa was released in 1999 it used the bait of adult website passwords (at the time much in demand) in a .doc file that was mass emailed. The Microsoft Word macro virus – actually caused a noticeable slowdown in Internet browsing – and an estimated £1.2 billion in damage. Numbers differ but some speculate the virus infected 20% of computers worldwide.

Code Red

A 2001 virus, Code Red targeted computers running a Windows 2000 and Windows NT. An infected computer displayed the message “Welcome to / Hacked by Chinese!” and was programmed, after two weeks, to launch a DDoS attack on major websites. Estimated damage? $2 billion and at least a million computers affected.


Conficker was first sighted in 2007 and was a simple enough proposition, downloading malware from predefined sites. However, despite its simplicity, the virus managed to do more than $9 billion in damage and was thought to have infected millions of machines.


Nobody loved this 2000 virus, as it was one of the most powerful in computer history, although it only infected 500,000 PCs. A text file email attachment masquerading as a love message, it even managed to get to computers at the Pentagon and was the reason why we now have laws against viruses. Estimated damage $15 billion.


Another mass mailer virus that infected more than two million PCs. SoBig.F caused $35 billion of damage slowing down global Internet speeds by stalling and crashing servers of all kinds in millions of systems. The virus was programmed to deactivate itself in 2003 (when it was released) and after that ceased to exist.


While worm MyDoom didn’t make any malicious changes to the computers it arrived in when it appeared in 2004 (other than to distribute itself via the address book), it spread at warp speeds, causing global traffic to slow down by 10% and generating $38 billion cleanup costs. Around two million PCs were infected.

Viruses are a serious issue in the digital age and infrastructure and security management is crucially important to business. Managed network services can provide a defence against network infection and also help to manage issues that occur to avoid lasting damage.