Antiviral utility developers today announced the recent discovery of a virus that infects HyperCard stacks. The "HC-9507" virus infects HyperCard's Home stack when an infected stack is executed, and from there spreads to other running stacks and randomly-chosen stacks on the startup disk. Depending on the day of the week and the time, the virus can cause odd system behavior when an infected HyperCard stack is running. For example, the screen may fade in and out, the word "pickle" may be inserted into your text, or the system may unexpectedly shut down or lock up.
Symantec and Datawatch have released updates to their SAM and Virex tools, respectively, which find and remove HC-9507 infections in HyperCard stacks. Check your documentation for instructions on obtaining the updates. Central Point Anti-Virus, Disinfectant, and VirusDetective do not attempt to deal with HyperCard viruses, so no updates are being released for these tools. Mac users who do not use HyperCard need not worry about this virus; only executing an infected HyperCard stack will spread the virus.