PID stands for Personal Identification. Every individual in the Net has a PID, which is stored in their icon. The back of an icon reveals the circuitry where the PID is stored. If a person gets ahold of another's PID, they can make that person do whatever they want. A person's PID must be on file for them to be registered online with a system's Principal Office.
Viruses must extract a person's PID in order to infect them. Superviruses are capable of infecting a person without direct access to their PID.