Virex: How Viruses Spread

A virus can become active when you launch an infected program, start your computer from a disk that has some infected system files, or open an infected document.

For example, if a game program contains a virus, the virus becomes active when you run the program. After a virus is in memory, it usually infects any program you run, including programs on 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 program is running. Turning off your computer or exiting the program 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 activates the next time you start your computer from the infected disk. If the virus resides in a program, the virus activates again the next time you run the program.

This information and more can be found at http://virexhelp.com