File-swapping on the Internet hit a sour note last week with the appearance of a virus that attacks users of the Gnutella file-sharing service. Security software vendors say it is the first virus to affect peer-to-peer communications. Named W32/Gnuman.worm, also known by the alias "Mandragore," the malicious file poses as an ordinary, requested media file. This masked file, however, is actually an .exe file that infects a user's computer once the program is run. After it infects a computer, the virus cloaks itself for other Gnutella users, leading them to believe that it is actually an MP3 music file or an image file.