#2. SoBig ($37.1 Billion)
In August of 2003, SoBig appeared, infections millions of computers across the world. SoBig evolved several times, making it hard to catch. It is a worm that replicates itself, but also is a Trojan, as it disguises itself as something other than malware. It caused an estimated $37.1 billion dollars worth of damage across the globe.

The SoBig viruses infected a host computer via e-mail attachment, using their own SMTP agent to gather e-mail addresses and spread itself. It was programmed to contact several IP addresses on August 26th 2003 and update itself. There is no clear reason why it was created. And, it even deactivated after only a month of operation. Its author is still unknown.