What is a primary characteristic of a virus?

A primary characteristic of a virus is its ability to replicate and distribute itself to other computers. Viruses are designed to invade systems and then replicate their code to spread to additional devices, often without the knowledge of the user. This self-replication mechanism is what differentiates a virus from other types of malicious software, allowing it to proliferate across networks and environments.

The other choices do not capture the essence of what a virus is. Antivirus software is specifically designed to combat viruses rather than being a type of virus itself. While some viruses may display advertisements as a form of malicious behavior, this is not a defining characteristic unique to all viruses. Additionally, the implication that viruses only affect mobile devices is inaccurate, as viruses can target a variety of platforms, including desktops and servers. The correct focus on replication and distribution is key to understanding how viruses function and pose risks to computer systems.