Virus - Code Red (2001)

Said to be one of the most expensive viruses in history, the self-replicating malicious code, 'Code Red' exploited vulnerability in Microsoft IIS servers. Exploiting the flaw in the software, the worm was among the first few "network worms" to spread rapidly as they required only a network connection, not a human opening like attachment worms. The worm had a more malicious version known as Code Red II. 

Both worms exploited a bug in an indexing service shipped with Microsoft Window's NT 4.0 and Windows 2000 operating systems. In addition to possible website defacement, infected systems experienced severe performance degradation. The virus struck multiple times on the same machine. 

Code Red II affected organisations ranging from Microsoft to the telecom company Qwest to the media giant Associated Press. According to a research firm Computer Economics, the virus caused damage worth above $2 billion. Incidentally, Microsoft had issued a patch to fix the vulnerability almost a month earlier, however, most system operators failed to install it.

a.abcnews.com said : Web sites affected by the Code Red worm were defaced by the phrase "Hacked By Chinese!" At its peak, the number of infected hosts reached 359,000.