A virus is a program that can "infect" other programs by modifying them and inserting a copy of itself into the program. During its lifetime, a virus goes through different stages which are: Dormant Phase- virus is idle waiting for some event to happen before it gets activated Propagation Phase- virus makes an identical copy of itself and attaches itself to another program Triggering Phase- virus starts performing the function it was intended for Execution Phase- virus does actual harm such as damaging programs and data files.