Life Cycle of a Virus

During its life time a typical virus goes through the following four phases.
1. Darment Phase-> The virus is idle.The virus will eventually be activated by some event , such as a date , the presence of another program or file , or the capacity of the disk exceeding some limit.Not all viruses have this stage.

2. Propagation Phase-> The Virus placed an identical copy of itself into other program or into certain system areas on the disk.Each infected program will now contain a clone of the virus , which itself inter a propagation phase.

3.Triggering Phase-> The virus is activated to perform the function for which it was intended as with the dormant phase the triggering phase can be caused by a variety of system events , including a count of the number of times that this copy of the virus has made copies of itself.

4.Execution Phase-> The function is performed .The function may be harmless , such as a message on the screen or damaging , such as the destruction of programs and data files.
Most viruses carry out their work in a manner that is specific to a particular h/w & s/e platform.Thus they are designed to take advantage of the details and weaknesses of the particular system.