The bug, also known as Novarg, was first detected on Monday night and had infected hundreds of thousands of computers around the world by Tuesday morning, said Finnish internet security experts.
"It's a traditional email worm that spreads very rapidly, also through the content of the Kazaa peer-to-peer network, and by this morning probably some 200,000 computers had been infected already," said Mikael Albrecht of the Finnish virus security firm F-Secure.
Kazaa is a popular file-sharing service that lets internet surfers share content such as games, movies and music with each other for free.
The bug also uses the email addresses stored on infected computers to replicate itself and spread further, said Albrecht.
Its main purpose is to attack and overload the web site of one of the world's biggest vendors of the Unix operating system, a competitor to Microsoft Windows, he added.
The bug's secondary function is to provide its author with a backdoor to the infected computers, enabling him to access and use them remotely, Albrecht said.
The virus might be linked to spammers -senders of unsolicited email advertisements - since such a backdoor function can be used to relay spam mails, he said.
The Mydoom virus is set to self-destruct on 12 February, but its backdoor function will work beyond that.