Perrun is the first to be able to cross from infecting a program to infecting data files.
Once run, the file installs a program onto the victim's hard drive that can infect pictures. When a computer user clicks on a picture file with the extension .JPG, the picture is infected before it appears. Because the picture displays normally the victim may not know there's anything wrong.
Perrun inserts portions of the virus code into the picture file. When the picture is viewed, it can infect other pictures. If the author wished, the virus could delete files on the computer or perform other mischief.
http://www.cnn.com/2002/TECH/interne....ap/index.html