A fork bomb for crashing system is a smallest writable virus as it contains on 5 Characters. Fork bombs count as wabbits: they typically do not spread as worms or viruses.
The concept behind the fork bomb: the processes recursively fork until a denial of service or a crash occurs. Fork Bomb creates a large number of processes very quickly and break the limitation of processes that can run on a system. It's dead simple: A program just replicate itself, which again replicate itself and so on until all resources are exhausted.
A fork bomb generally creates two instances at startup, further each of created processes creates two instances and so on…The processes recursively fork; this “forks” the processor and jamm it completely until a crash occurs. It use forks to do system crash, so its called Fork Bomb.
See, How to create it:
1. Fork Bomb for windows:
Write the following code in notepad and save it with bat extension say fork.bat
%0|%0
Once anyone will do double click on this bat file, his system will jamm.
2. Fork Bomb for linux/Unix:
:(){ :|:& };:
[Note: Be careful in using these scripts, i will not be responsible if you damage your system.]