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Macintosh Viruses
What Viruses Do
Some viruses are programmed specifically to damage the data on your computer by corrupting programs, deleting files, or erasing your entire hard disk. Many of the currently known Macintosh viruses are not designed to do any damage. However, because of bugs (programming errors) within the virus, an infected system may behave erratically.
What Viruses Don't Do
Computer viruses don't infect files on write-protected disks and don't infect documents, except in the case of Word macro viruses, which infect only documents and templates written in Word 6.0 or higher. They don't infect compressed files either. However, applications within a compressed file could have been infected before they were compressed. Viruses also don't infect computer hardware, such as monitors or computer chips; they only infect software.
In addition, Macintosh viruses don't infect DOS-based computer software and vice versa. For example, the infamous Michelangelo virus does not infect Macintosh applications. Again, an exception to this rule are the Word and Excel macro viruses, which infect spreadsheets, documents and templates which can be opened by either Windows or Macintosh computers.
Finally, viruses don't necessarily let you know that they are there - even after they do something destructive.
How Viruses Spread
Viruses spread when you launch an infected application or start up your computer from a disk that has infected system files. For example, if a word processing program contains a virus, the virus activates when you run the program. Once a virus is in memory, it usually infects any application you run, including network applications (if you have write access to network folders or disks).
Viruses behave in different ways. Some viruses stay active in memory until you turn off your computer. Other viruses stay active only as long as the infected applications is running. Turning off your computer or exiting the application removes the virus from memory, but does not remove the virus from the infected file or disk. That is, if the virus resides in a system file, the virus will activate the next time you start your computer from the infected disk. If the virus resides in an application, the virus will activate again the next time you run the application.
