What is the primary function of a virus?

Answer 1

Viruses are malicious software (malware) in which their sole purpose is to replicate and infect non-infected computers. Their are many mediums to which a virus can infect a computer, most commonly through the internet and email attachments.

When viruses infect a computer, it attaches itself to a "host" file (hence the name virus because it acts like a real world virus). Next, it injects its code into the host file so that whenever that infected file is run, the virus executes first and then the actual file starts. The "payload" is the official term to the effects a computer virus has on a PC.