A File Infector Virus (also called a file-injecting or parasitic virus) is a type of malware that attaches its own code to executable program files, typically those with extensions such as .exe or .com. It lies dormant inside the host file and is triggered only when the user runs that program, which is exactly the behaviour described in the question.

Once the infected program is launched, the virus code executes first (or alongside the program) and can:

• Copy itself into other executable files on the same computer, spreading the infection.
• Corrupt, overwrite or delete data.
• Degrade system performance or open the door to further malware.

Because activation depends on the host file being executed, this class of virus needs a host program to survive and spread, distinguishing it from self-standing malware.

The other options behave differently. A Boot Sector Virus infects the master boot record or boot sector of a disk and loads when the computer starts up, not when a program is run. A Macro Virus hides in the macros of documents such as Word or Excel files and triggers when the document is opened, targeting data files rather than executables. A Worm is not strictly a virus at all: it is self-replicating and spreads across networks on its own, without needing to attach to or wait for a host file to be executed. Since the defining clue is attachment to executable files with activation on running them, the correct answer is the File Infector Virus.