The way a virus or rogue program spreads to other computers varies with the type of virus.

• A program virus becomes active when a program infected with a virus is run. Once a program virus is active, it will usually infect other programs on the computer. If a copy of an infected program is moved to and run on another computer, it can then infect programs on that computer.
• A boot sector virus infects hard disks and diskettes. If a computer is re-booted or its power is turned on while an infected diskette is in its disk drive, the virus will spread to the hard disk, even if the diskette is not capable of starting up the computer. Once the hard disk is infected, all diskettes used in the computer will be infected and can spread the infection to other computers.
• A macro virus becomes active when a document infected with the virus is opened using the program it is designed to attack. The program must have its ability to run macros enabled (turned on). Generally, when a virus in a document becomes active, it will spread to global settings for the application so that other documents will become infected when they are opened. When an infected document is opened on another computer, the global settings used by that copy of the application will be infected as well.
• An e-mail virus spreads through e-mail messages and usually replicates by automatically mailing itself to all entries in the victim's e-mail address book.
• A worm replicates itself using existing security holes on systems attached to a network. It scans the network for for computer that will let it exploit the security hole to infect them. Then, from the newly-infected computers, it continues to probe the network for other vulnerable systems.
• A Trojan horse cannot usually replicate itself. However, it, too, can still damage your computer.