The first stage is called Pre-trigger stage or the dormant stage. In this stage viruses lie dormant, and does not do any destruction. (This act is also similar in the biological virus.) It is hard to detect a virus in this stage.
The second stage called Trigger stage is the one in which virus performs any destruction. A trigger can be made to set off at a given time, given number of times a program is run, physical condition of disk, specific date or time, any other event or just anything which might have been thought of by its developer.
Once this trigger goes off, the destructive action mentioned in the virus program executes to carry out the destruction. This is said to be the final stage as it causes actual damage it is supposed to do.Virus program enter into the system either by way of copying the carrier programs (exe,com, bat, sys & similar files) or copying anything from a disk with infected boot sector or partition table or even through E-mail or Website contents. Also, the replication activity of viruses is transparent to the user.