Algodoo

The virus is the orange box. Every object that collides with it will have it's onCollide and postStep changed to the virus's. Instead of writing a set program out to write to the collided block's code, it has to directly copy ALL of the code of onCollide and postStep. Otherwise the code of the parent virus would have to be infinitely long.
Think of it like this. If you told someone a secret like "Abc, pass it on" you would technically need to mention they need to tell their reciever to pass it on, and tell them they need to tel their reciever that they need to tell their reciever to pass it on, and so on.

Thanks.
it is important to mention that you must drag the parts around!