A new disease is passed throughout the population.

People contract the disease and then pass it along to others. Eventually a simple cure is developed and made available at a clinic. Those who are cured pass along the simple cure to others. 

What are the two stocks in this system? the flows? Draw a stock and flow diagram depicting the system.
Run the model for 5 year with the following values: (Disease.mdl)
 

Each person who has the disease spreads it to two other people per year.

A cure is discovered and offered at a clinic after year two.

At the beginning of year three, a dozen people are cured at the clinic each year

Each person who has been cured, spreads (tells) the cure to four others each year

Note: The initial number of people who contract the disease is one.

 

Now simulate the system for 6 years. Check for reasonableness.

Now simulate the system for 7 years. Check for reasonableness. What happens to the number of people have the disease on the 7th year? Can a stock of people be negative in a real system?
 

Why does it become negative? Compare the number leaving the stock with the number in the stock plus the number entering. In a real system can more leave a stock than the sum of those entering and those already there?

Change the model to place a limit on the number leaving the stock so the the level of the stock can not be less than zero.
(Save the changed model as DiseaseConstrained.mdl)

      
This is the end of the exercise. When you have completed the exercise, show the lab instructor your final graph in lab and submit DiseaseConstrained.mdl to Blackboard.