How do computer Virus spread?

How does a computer virus spread?

Computer viruses spread from one computer to another by the transfer of infected data

through email, pen drives, or other sources.

Viruses become active when you start an infected application or start your computer from a disk that has infected system files. Once the virus is in the computer's memory, it usually infects any application you run. Different viruses behave in different ways.Some stay active infected application is running.