Definition:
Malicious software program that when triggered, replicates itself by modifying other computer programs. A virus can only spread itself on the local machine.

Virus Phases:
Dormant phase - virus is on the system but does not take action and is waiting for a “trigger” to activate.

Propagation phase - Virus is replicating itself. Places copies of itself into other programs or system areas.

Triggering phase - multiple things can trigger a virus including user interaction or the virus copying itself “x” amount of times before activating.

Execution Phase - Actual work of virus where the payload is released.

Example:
The Melissa virus was a email attachment virus that when opened causes the virus to be resent via Microsoft outlook to the first 50 people in their address book. It would also disable safeguards on Word 97 & 2000.