Additional information (hy)
Hypercard virus
MerryXmas!
Some recources claim that this was the first hypercard virus, actually: that's not the case (Dukakis was the first, in 1988), but it was very well-known and stayed around for years. MerryXmas was released in 1991, (written in Hypertalk-language).
When an infected stack was opened, the virus script copied itself to the Home stack. From there, it spread to every other stack that was opened. In 1991 it was also able to infect a Mac-pc.
This type of viruses can cause unexpected Home stack behavior. They can shut the system down without saving open files and display messages and plays sounds. Hypercard-viruses are known to often have many variants (with totally different names) and were hard to get rid of.