During the infection stage, the virus usually replicates itself, conceals itself and activates only when the necessary conditions are found.
Each of these actions assures a certain mission:
- The virus replicates to make sure it stays in all areas of the computer for the longest possible time
- Conceals itself for its safety
- Activates only when necessary conditions occurs to does the damage it's supposed to do.
Therefore, you must always protect your computer by using a strong antivirus and frequently scanning the computer.