The Merry Christmas (merryxmas) virus is written in HyperTalk, the HyperCard scripting language. It can’t affect any files other than HyperCard stacks and does not destroy or damage data. The most common indication of the merryxmas virus is that stacks launch and run much slower.
Many stacks can function properly when infected, but the Service Source won’t function properly with this virus present. When the Service Source CD-ROM is built, every stack on the CD is checked for the virus, which is eliminated if found.
Apple had a fix for this:
This vaccine stack will quickly locate all the stacks on the volume of your choice; peek at the stack scripts (without opening the stacks); and replace the infected scripts without the virus code. A bonus feature is the option to let it compact those stacks that have free space available for compacting. Don’t worry about locked stacks — this stack will unlock and relock them.
Read more about Merryxmas and other HyperCard issues here.