Wonder Bar details
How does the Wonder Bar object cure the disease ?
Your Sims can fall sick e.g. after being bitten by their guinea pig. If not cured, they can die within a few days.
The sickness is an invisible object, which was placed on the lot just after the guinea pig bite. This object controls behavior of your Sim - it decreases comfort and energy very quickly and makes your Sim cough. It also contains the state of the disease with several steps - when reaching the last step, a request of death is placed into the Sim's action queue.
Using the Miracle Elixir will launch an action - searching for instance of the sickness object on the lot and if one has been found, it resets the disease steps and finally it removes the object from the lot. That's why only one use of the Miracle Elixir cures all the Sims in the family.
Here you can see how the programming of objects looks like - using the IFF Pencil 2 tool:
Have fun with my stuff :o)