In this introduction to the S-I-R Model, students explore a scenario concerning the spread of an infectious disease. They use the S-I-R model to determine the number of people infected, how many will recover, etc. Students write an equation for the number of people who will become infected. In small groups, they develop IVPs for given situations which will modify their S-I-R model. This four-page learning exercise contains approximately five multi-step problems.