Vincent Gullotto, director of Network Associates' McAfee antivirus emergency research team, said the first sample of the new strain came to his lab about a month ago. In the past day or two, however, more than two dozen reports have come in.
Like other Melissa variants, the bug's main impact is that it tries to replicate itself. Future Microsoft Word documents created by infected systems can carry the virus and, when the virus is a Windows-based PC running Microsoft Outlook, it will mail 50 copies of itself using the program's address book.
Dubbed alternatively Melissa-X and Melissa 2001, it can be spread on either a PC or a Macintosh. Like other Melissa variants, though, only Windows PCs will send mass e-mail copies of the infected file. For now, the virus attachment has appeared as "anniv.doc," although Gullotto cautioned this could change.
Although the newest virus is no more virulent than other Melissa strains, virus software has to be updated to recognize a document as infected with the new strain.
Gullotto said that because it is a strain of Melissa, the new variant probably ranks as a high risk, although by itself it would probably rate as only a medium risk so far.
"We have a concern about the virus, as we do about most viruses," Gullotto said. "We don't want to alarm people at this point."
While it is hard to say precisely how the new variant came about, Gullotto said it is possible someone saved a Melissa-infected document into the new Macintosh version of Microsoft's Office software, creating the new strain. Both Macintosh and Windows computers can open Office documents created on either computer, he said.
Gullotto said he sees about one new strain a month of the Melissa virus, which first cropped up in March 1999.