The writers behind a recent rash of virus may be building a super-worm, according to security researchers.

The "HellBot" group behind the Mytob worms have included programming instructions in their code that mirror the way developers work, warned Sophos security consultant Carole Theriault. "The only conclusion we can come up with is that they are working on a big super worm," Theriault said.

Since its discovery in February, the Mytob mass-mailing worm has spawned dozens of variants, each slightly different, according to researchers. However, each variant turns off an infected machine's security settings and blocks the user's access to security websites, Theriault said.

This makes it difficult to get help once a machine has been infected, and Trojan programs accompanying the worms could leave a backdoor open for attack, she said. One recent version included spyware and adware, which could be used to reap monetary benefits, according to anti-virus company Trend Micro.

The Mytob authors have been "very busy", releasing multiple variants a day, McAfee noted. While the distribution of each variant is low, combined there is a lot of activity around them, researchers said.

Over 50 percent of the reported problems coming into Sophos over the last 24 hours have been about Mytob worms, Theriault said. Recent versions, discovered earlier in the week, include Mytob.bi, which poses as a message from an IT administrator, warning that the recipient's e-mail account is about to be suspended.

It scans the hard drive of an infected machine and sends copies of itself to e-mail addresses it finds in the Windows Address Book. It also prevents the machine from accessing several anti-virus and security websites, and can open a random port, allowing a hacker to gain remote access.

While anti-virus companies would normally have to update their software to guard against each new variant, the Mytob family is so close that multiple variants can be caught using generic definitions of the worm, Theriault said. However, users are advised to keep their anti-virus software up-to-date.