The exact details depend on the type of virus. In general here’s what happens: The first step takes place when you run the infected program; the virus is activated and after a period of time infects your System. The next time you start your computer the virus is already there and ready to spread. When you run an uninfected program the virus may then spread to it. Eventually the virus will spread to all the applications you run.
-- part contents for background part 11
----- text -----
Once the System is infected and the virus starts spreading to other programs, it will do whatever nasty thing it was designed to do. Some only display messages, others do worse things!