Which of the following best describes a virus?

A virus is best described as malware that spreads and infects systems. This definition captures the essence of what a virus is: a malicious software program designed to replicate itself and spread from one computer to another. Viruses can attach themselves to legitimate programs or files, and when these infected programs are executed, the virus activates and can cause harm to the system, compromise data, and disrupt normal operations. This behavior distinguishes viruses from other types of software, as their primary function is not to perform helpful or protective tasks, but rather to cause damage and propagate themselves within computer networks.

The other options present definitions related to different types of software or intentions. For example, secure communication tools are not related to malicious software, while programs that protect computers are classified as antivirus software, designed specifically to combat threats including viruses. A harmless software application also does not align with the core definition of a virus, which inherently involves harmful intent and actions.