Once a worm is executed it seeks other systems rather than parts of systems to infect, and then copies its code to them. Some viruses display symptoms and some cause damage to files in a system they have infected but neither symptoms nor damage are essential in the definition of a virus. A non-damaging virus is still a virus and not a prank. A virus is surreptitiously self-installed. It may modify other software in the system without user awareness, and removal can be difficult and costly.
Many viruses cause intentional damage.
But many more cause damage that may not have been intended by the author of the virus. For instance, when a virus finds itself in an environment very different than that for which it was written it can suddenly become very destructive. Even if a virus causes no direct damage to your computer, your inexperience with viruses can mean that damage occurs during the removal process. Many organizations have shredded floppies, deleted files, and done low-level formats of hard disks in their efforts to remove viruses. The most common infections are boot viruses, even though the majority of the viruses in the world are file viruses.