A file infector is malware that copies itself into executable files, so infected programs become carriers for the malicious code. When the user launches the program, the malware runs too. This is different from malware that lives only in memory or on a server: the infection can persist inside files already stored on disk.
File infectors matter because cleanup is harder than simply blocking network traffic. Even if defenders disrupt command-and-control, infected executables may remain on endpoints and can reintroduce the malware later. In real attacks, file infectors can spread by modifying many programs on a system or network share, making trusted software part of the infection chain. Defenders usually need both network containment and endpoint remediation, including scanning, quarantine, and replacement of altered executables. In botnet investigations, this is why cutting off peer communication may not be enough on its own.