File infector virus
Virus that infects other files on a system or network
File infector viruses are the 'classic' form of virus, those to which the term is most commonly and, along with boot sector viruses, most appropriately applied.
When an infectious file is executed on a system, the infection routine will seek out other files and insert its code into them, generally at the beginning or end of the existing file (prepending or appending viruses), but also occasionally in the middle of the file (mid-infector) or spreading itself across gaps in the file structure. The entry point of the file is redirected to the start of the virus code to ensure that it is run when the file is executed, and control may or may not be passed on to the original program in turn.
File infector viruses often misinfect, either leaving the file completely non-functional or simply failing to run the viral code at all. More sophisticted forms of file infector virus, which try to hide their presence by changing aspects of their code with each infection, are known as polymorphic or metamorphic viruses.