This virus, which first appeared in late 1989, is the first virus which can spread from an infected disk without any action by the user. It is named after the Window DEFinition resource it uses to infect computers.
The WDEF virus infects the invisible
“Desktop” files used by the Finder. Every Macintosh disk has one of these files (hard disks and floppies). The virus spreads from Desktop file to Desktop file, but it does not infect applications, data files, nor system files.
The virus does not intentionally try to do any damage. In fact, it doesn't do anything except spread from disk to disk.
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Due to a bug, the virus causes Mac IIcis to crash. It has also been noted to cause unusually frequent crashes on infected Mac IIcxs, and severe performance problems with infected AppleShare servers. There are also other bugs in the virus which could cause problems.
One visible symptom of the virus is problems with the font used by your menus.