The usual virus checking procedures did not detect the virus because it was so new, the magazine said.
"This variant got developed at almost exactly the same time we were pressing the disc, so none of our virus checking software was [able to catch it] at that point" said Cheryl England, publisher and editorial director of MacAddict.
England said that a letter was inserted with the issue, alerting users to the problem with the disc and how to prevent the virus from infecting their system. She said the company has also extended every subscription by one month; the company had 158,000 audited subscribers at the end of June.
The magazine posted a message on its Web site about the problem.
Virus checking programs typically use lists that characterize known viruses and update these lists frequently; however, a new virus that crops up between list updates can slip by, undetected.
In the past, Apple's Macintosh computers have been remarkably free of destructive viruses compared to Windows-based computers, but the infection is yet another piece of evidence that the resurgence of the Mac platform is generating interest from unsavory characters (See related story).
"It used to be the Mac was pretty immune to viruses. These things are cropping up with alarming regularity now, so everyone has to be more diligent in checking these things," said Colin Crawford, president and CEO of rival Mac Publishing LLC, which produces the Macworld and MacWeek.com publications. MacAddict is published by Imagine Media.
Earlier this week, a report surfaced that a CD-ROM Macworld shipped out with its most recent issue was infected with a virus, but Crawford said that the company has completed an investigation and has not found any virus present on the disc.
Worms are described as programs that copy themselves as they move from file to file, whether they came from floppy disks, hard drives, removable storage units, or the system's main memory. They differ from a normal virus in that they don't need to be attached to a program or document to replicate themselves.
On the Mac, the AutoStart worm program attaches itself to the settings control panel for QuickTime version 2.0 or later on PowerPC-based systems. After doing this, an infected computer will suddenly begin to destroy data in a variety of files.
The problem can usually be prevented by turning off the AutoStart setting, which is used to automatically load in CD-ROMs, according to Mac Virus, a Web site devoted to tracking viruses on the Mac.