A malware that infects multimedia files has been recently discovered. The malware modify the media file .WMA (Windows Media Audio) such that they download a fake codec when played. When a user tries to play an infected file, a popup message is displayed, asking the user to download a certain codec in order to play the file. The downloaded codec is of course, nothing else but a malware.
The malware has capablities to convert other formats like MP2, MP3 to .WMA and then infect.
People normally keep thousands of multimedia files on their systems, especially MP3s. If each file is infected by the malware then shared through a P2P network, then the user unknowingly turns into a malware host.