26 April 1999
On 26 April 1999, the CIH computer virus infected a large number of computers around the world with Korea bearing the brunt at over $250 million worth of damage. The virus became known as Chernobyl, so named because its outbreak coincided with the thirteenth anniversary of the Chernobyl reactor meltdown. It would infect the BIOS chip in a computer and render the whole device completely useless. The virus was authored by a Taiwanese man Chen Ing-Hau, who was later detained. The virus is no longer a threat as it can only affect older operating systems like windows 98 and earlier versions. Lately viruses have moved from data destruction to information extraction, such as credit card information.
References

Sophos (2004) Fifth anniversary of Chernobyl computer virus attack [online] Available at: www.sophos.com [Accessed on 12 April 2013]|

Wise Geek What Is the Chernobyl Virus? [online] Available at: www.wisegeek.com [Accessed on 12 April 2013]