The worm is particularly difficult to root out, because it lands in e-mail boxes using a number of different subject lines, such as "re: details" or "re: here is the document."
"It arrives with an attached PIF (program information file) file, and it's already extremely widespread," said, senior technology consultant at Sophos.
When opened, the virus PIF file will rapidly replicate itself, slowing down computers and e-mail bandwidth.
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"We suspect people are more laid back about PIF files, because they may not have heard of them and may not realize they can contain dangerous code," Cluley said. "The best thing to do with this file is to delete it. Don't open it."
, was rated the third-worst computer virus in February after MyDoom.A and Sober.C, according to Sophos, which writes antivirus and antispam software.