A "virus" is a type of malicious software or code that is illicitly introduced into a computer or network device. Once inside the system, a virus can replicate itself and spread to other files or devices. It can be designed to damage or disrupt normal computer and network operation. Viruses often require user interaction to spread, such as opening an infected email attachment or executing a malicious program.
In the context of the given options:
'Protocol' refers to a set of rules that define how data is transmitted over a network.
'Catastrophe' generally refers to a sudden and widespread disaster, not specifically a type of malicious code.
'Medium' typically refers to the physical or virtual means by which information is transmitted, but it doesn't specifically describe illicitly introduced code.