What is a key characteristic of a computer virus?

A key characteristic of a computer virus is its ability to replicate itself within other programs. This self-replicating behavior allows the virus to spread and infect additional files or systems without the user's consent or knowledge. When a user executes an infected program, the virus may take action to attach itself to other programs or files on the same system or network, enabling it to propagate further. This mechanism is what differentiates viruses from other types of malware, as their primary mode of spreading is through the integration and modification of existing legitimate software.

The other options describe misconceptions about computer viruses. For instance, while viruses can spread through various methods including emails, they are not limited to just this means; they can also be transmitted through removable drives, downloads, and network shares. The assumption that a virus is always detected by antivirus software is inaccurate, as some viruses can evade detection, especially if they are new or use techniques to mask their presence. Finally, the assertion that a virus enhances system performance is incorrect; in reality, viruses typically degrade system performance, cause data loss, or interfere with normal operations.