While rare, it is possible to infect actual programming source code found on your computer.

Source code comes in many forms because of the many different types of compilers and languages available. This is one reason why source code viruses are not particularly common. The other is that so few people actually write programs it becomes difficult for a source code-only virus to find victims to infect.

Also, because of programming style and differing designs that individuals use when they write program code it's difficult to write a virus that actually spreads via this mechanism. More typically, a source code virus will not infect via source code but simply add Trojan material to existing source code so that when it is compiled and run it does something different than expected.

Die Hard is one example of a type of source code virus. The virus actually spreads by infecting COM and EXE files (a file virus) but, as part of its payload, in drops Trojan code into any ASM (assembly language) and PAS (Pascal) source files as they are accessed (when the virus is resident in memory).

Source code viruses are not common; but be aware they do exist and have been seen in the wild in the past.

Added 28 Aug 2009: This virus type has once more reared its head in the Delphi W32/Induc virus. This virus is designed to infect the source code of Delphi which is a variant of the Pascal language originally developed by Borland.

Summary

• Source code viruses add instructions to existing programming code found on your system.
• They are rare and the code they add is typically a Trojan instead of a full virus.

What Viruses Infect
Batch Files	Visual Basic Files