Mydoom, a new computer virus spreading by email, is breaking records for new infections, antivirus vendors and security companies say.

Infected email messages carrying the Mydoom virus, also known as "Shimgapi" and "Novarg," have been intercepted from over 142 countries and now account for one in every 12 email messages, according to Mark Sunner, chief technology officer at email security company MessageLabs.

That surpasses the Sobig.F virus record, which appeared last August and, at its peak, was found in one of every 17 messages intercepted by MessageLabs.

Since first detecting the new virus at 1pm on Monday, MessageLabs intercepted almost one million infected email messages carrying the virus.

The virus has "followed the sun," hitting hard in the US and Canada late on Monday, then working its way through Asia and Europe on Tuesday.

F-Secure of Helsinki estimates that around 100,000 computers have been infected with Mydoom so far, says Mikko Hypponen, manager of antivirus research at F-Secure.

Tech talk

The worm arrives as a file attachment in an email with a variety of senders and subjects, such as "Hello," and "test." The message body is often technical sounding, imitating the look and feel of an automatically generated message from an email server.

For example, some email messages telling recipients that "the message contains unicode characters and has been sent as a binary attachment," or "The message cannot be represented in 7-bit ASCII encoding and has been sent as a binary attachment."

Users who click on the attachment, which uses a variety of file extensions such as ZIP, SCR, EXE, and PIF, are infected with the virus.

The technical pitch is a new twist on so-called "social engineering" techniques used by virus writers to trick users into opening malicious file attachments. Mydoom's authors may have been counting on the fact that people trust the authenticity of computer generated messages more than those purporting to come from other humans, according to Sunner.

While Mydoom has shattered Sobig.F records, in many ways the two viruses are the same, antivirus experts agree.

Both viruses scan infected computers for email addresses that are then targeted by infected email. Also, both Sobig.F and Mydoom are small and contain highly efficient SMTP engines for sending out copies of themselves. The efficiency of their mail engines means that even a small number of infections can generate a massive amount of email traffic.

Finally, both Sobig.F and Mydoom contain a Trojan horse program that gives remote attackers full control of the infected system.