After they are finished loading, they load the original boot code, which they have previously moved to another location. File viruses attach to executable program files in such a way that when you run the infected program, the virus code first executes. After the virus is finished loading and executing, it loads and executes the program it has infected. Macro viruses attach their macros to templates and other files in such a way that, when an application loads the file and executes the instructions in it, the first instructions to execute are those of the virus. A companion virus attaches to the operating system, rather than files or sectors.
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, 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.