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.
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.