There are four stages of a virus (inspired by biologists' classification of the life cycle of a real virus)

The dormant stage: this is when the virus hides in your system and waits.

The propagation phase: this is when the virus spreads - it makes copies of itself, storing copies of itself in files, programs or other parts of the disk. Clones may change slightly to avoid detection, and they will also replicate themselves, creating more clones that continue to replicate themselves, and so on.

Trigger phase: A specific action is usually required to trigger or activate the virus. This may be a user action, such as clicking an icon or opening an application. Others are programmed to resurrect after a certain amount of time, for example after your computer has been restarted ten times (this is done to confuse the source of the virus).

Execution phase: The "fun" begins. The virus releases its payload, i.e. the malicious code that damages your device.