This is a virus which attaches itself to program files. It infects any program for which execution or interpretation is required.

File infectors can be either 'Direct Action/Non-Resident' or 'Resident'. The former virus selects one or more programs to infect each time a program infected by it are executed. The resident virus installs itself somewhere in memory (RAM) the first time an infected program is executed, and thereafter infects other programs when they are executed or when other conditions are fulfilled. Most viruses are resident.

This virus infects both the files and boot sectors.

This virus modifies directory table entries so that the virus is loaded and executed before the desired program is. The program itself is physically intact, only the directory entry of the program file is infected.

This virus targets specific features of the programs that contain the core or kernel of an operating system.

This virus hides the modifications it has made to files or boot records, when activated. As a result programs that try to read infected files or sectors view the original, uninfected form instead of the actual, infected form, leading them to go undetected.

This virus produces varied but operational copies of itself with the aim of avoiding detection under scanners.

This virus copies itself to memory when a program infected by it is executed, and then infects other programs, not only ones being executed but also those that are merely opened. In which case, running a scanner can result in all or many programs becoming infected.

This virus infects less often, only occasionally or only files whose lengths fall within a narrow range, etc. It thus minimises the probability of being discovered.

This virus creates a new program, which unknown to the user, is executed instead of the intended program.

This virus uses special tricks to make tracing, disassembling and understanding of its code difficult.

This virus overwrites a part of the host file that is filled with a constant, without increasing the length of the file, but preserving its functionality.