Retro viruses sometimes try to avoid corrupting anti-virus software while in the infection phase. They do this because, if the anti-virus software is not working properly, the user will attempt to fix it. This would make it difficult for the virus to continue spreading. File Viruses File viruses are viruses which generally attach themselves to regular program files. They can attach to various locations of the original file, replace code, fill in open spaces in the code, or create companion files to work with an executable file.
File viruses fall into different classes depending on which method they use to infect.
File viruses usually infect EXE programs, though some can infect other kinds that can be executed. Most of the file viruses are memory resident and wait in the memory until the user runs another program. While another program is running, the virus replicates. Parasitic viruses are a type of file virus. As the name might suggest, parasitic viruses attach to or insert into a file, but the file remains usable.