Symantec has created software to simulate the outbreak of a worm. According to the article published by Symantec:
The new Worm Simulator takes VBSim to another level, enabling custom configuration of new worm simulations, configuration of custom networks and protection policy, and incorporates impressive new three-dimensional graphics.
Currently, the simulator can represent Blaster, MyDoom, Netsky, Sasser, Slammer, and SoBig. I emailed Adam Bromwich from Symantec asking if it would be possible to create our own simulations, and he responded:

Unfortunately the ability to create worms and networks is
restricted in the Worm Simulator. New worms and simulations can be created
only by Symantec in this first version. We are considering providing tools
to users so they can create their own simulated worms and networks.
The software is fun, but it won't be of much use until it's customizable.

This is based on VBSim, which is pretty neat as well. Can be found here: