Objective: Students will understand the dynamics of the transmission of diseases by taking part in a "hands-on" simulation.

Background: Students are completing a unit on cell biology, including the structure and function of the organelles, differences between prokaryotic and eukaryotic cells, and the properties of the cell membrane. The topic of infectious diseases has generated many questions among the students and has been used to illustrate several processes. For example, the marker proteins of the cell membrane give cells their identity. When disease causing organisms with different marker proteins enter the body, they are targeted for destruction by the white blood cells. To complete the cell unit, I have designed an activity that is useful in teaching students how diseases are transmitted, and one which makes them feel a part of the process. This is also a good lesson to get students thinking about their vulnerability to sexually transmitted diseases, and leads naturally into a unit on disease.

Introduction: Begin with a discussion of how epidemics begin, and how they spread. Give some examples from history, such as Typhoid, the Plague, AIDS, and Ebola, or make reference to recent movies such as "Outbreak". Tell students the story of Typhoid Mary, and describe the role of the CDC (Center for Disease Control). Discuss the concept of a quarantine. Explain how today's simulation will work.

Materials: Prepare a collection of Dixie cups or (even better) clear plastic cups. You should have one for each student. In one of the cups, put sodium hydroxide (NaOH), a clear colorless liquid. In each of the other cups, fill to the same level with tap water. Put a secret mark on the cup with the sodium hydroxide, or note carefully which student takes the unique cup. You will need a dropper bottle with phenolphthalein indicator solution later in the lab. Phenolphthalein is an organic compound (C₂₀H₁₄O₄) used as an acid-base indicator. (Interestingly, it is also the active ingredient in ExLax!) The compound is colorless in acidic solution and pinkish in basic solution (with the transition occuring around pH 9). These preparations must be made before students enter the room. Warn the students not to spill the contents of the cups and to wash immediately if they come into contact with the liquid.

Procedure: Write down on the board the names of all the students in the class who are present today. Have students copy this list of names onto the handout of names. The cups with liquid represent bodily fluids, and students will mix their bodily fluids to simulate the spread of a disease. Exchanges will occur in two separate rounds, which we will call "Day 1" and "Day 2". Students will each select a person with whom to exchange fluids. When everyone is done, Day 1 is over and Day 2 begins with a second round of fluid exchange. Therefore, each student will be a "giver" exactly twice, but the number of times each student is a "receiver" will vary. Exchanges will occur in two separate rounds, which we will call "Day 1" and "Day 2". When completed, ask each student (the giver) who their two receivers were, so all students can get the data copied onto their sheet.

Diagnosis & Analysis: Add a drop of indicator solution to each student's cup. If the solution remains clear, they are healthy. If the solution turns pink, they are infected. Cross out all of the names of students who came into contact with the disease, and ask them to try to figure out who was the source of this deadly disease. Tell them that only one person was initially "infected", and that the best clues will come from looking at people who exchanged fluids with a sick person, but who are not sick themselves. This will indicate that the sick person contracted the disease after that contact, and also shows that this person was not the source of the infection. Insist that students explain the path of infection rather than just guess who was the source. Finally, reveal the source and have students see if they can then trace the path of infection.

Timeline: 1 hour

15 min. Introduction of the disease simulation and copying of names.

05 min. Fluid exchange Round 1- spreading of the simulated disease.

05 min. Fluid exchange Round 2- spreading of the simulated disease.

05 min. Recording and copying of fluid exchange data to and from the board.

10 min. Determination of the infected individuals while students begin work on lab questions.

05 min. Listen to student theories, and ask for evidence.

05 min. Announcement of the infectious individual, and explanation of the results.

10 min. Continued work on the lab questions, and time for more discussion.

List all of the students in the first column. After two rounds of "bodily fluid exchange" record both contacts and share the data. After the data is recorded, the teacher will add an indicator which tells who lived and who died. You must then try to recontruct the path of this epidemic back to its single source.

1. How would the results differ if you exchanged fluids with more/fewer people in class?

2. How would the results differ if the class was larger/smaller but the number of contacts was held constant?

3. How would the disease spread if you only exchanged fluids with people you know well? Name a disease that is spread in this way.

4. How would the results differ if you have only a 50% chance of contracting the disease after being exposed?

5. How would the results differ if people were continuously entering and leaving class?

6. How would the results differ if the infected person dies very quickly/very slowly after contracting the disease?

7. Which disease will be more evolutionarily successful-one that kills quickly or one that kills slowly? Why?

8. How would the results differ if the infectious person shows visible symptoms?