Modeling the Transmission of a Communicable Disease

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

Introduction: Begin with a discussion of how epidemics begin, and how they spread. Give some examples from history, such as the Plague, AIDS, Ebola, H1N1, or make reference to movies such as Outbreak. Tell students, or have them listen to, the fascinating story of Typhoid Mary, and describe the role of the CDC (Center for Disease Control). Discuss the concepts of a biohazard, quarantine, epidemic and pandemic. Explain how today's simulation will work. Ask why local epidemics can more easily become pandemics in the modern world.

Simulations

https://www.learner.org/courses/envsci/interactives/disease/disease.html

https://science.education.nih.gov/supplements/webversions/InfectiousDiseases/activities/activity4-flash.html

http://www.gleamviz.org/simulator/

http://fred.publichealth.pitt.edu/proj/measles/

http://www.shodor.org/interactivate/activities/SpreadofDisease/

Materials

Option A-More Dramatic: Prepare a collection of clear plastic cups. You should have one for each student. In one of the cups, put a sodium hydroxide (NaOH) tablet dissolved in water to create a clear colorless liquid with a high pH. 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 pH 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 laxatives!) 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.

Warning: Students should be careful not to spill the contents of the cups and to irrigate the affected area immediately with water if they come into contact with the liquid, as it can cause mild irritation to the skin and eyes. Only add a small amount of NaOH to water. Never add water to a large supply of NaOH. The reaction is exothermic (it gives off heat) and could boil a small amount of water rapidly. Although it might seem obvious, DO NOT DRINK any of these fluids!

clear cupred liquid in cup clear cup

Option B-Cheap and Easy: If the chemicals are a concern, or are difficult to obtain, you can modify this lab with the use of opaque cups and food coloring, but you'll have to make a few adjustments. The infected person has a cup with water and a lot of dark blue or dark red food coloring, and everyone else has a cup with just plain water. The cups should be opaque rather than clear (so people can't easily see who's infected), and all fluid exchanges should be conducted secretly so that nobody knows whether they are about to encounter an infected person or a healthy one (keep your cup covered with your hand so they can't see if you're infected!). Then proceed as before, with several rounds of fluid exchange, and gather your data at the end on who is infected. Do the fluid exchanges in total silence so as not to give the answer away. Have the uninfected people try to figure out who was the source (because the infected people will know when it happened).

Procedure: Write down the names of all the students in the class who are present. 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. When completed, ask each student (the giver) who their two receivers were, so all students can get the data copied onto their sheets.

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. (Alternately, with Option B, any cup with reddish colored liquid is infected, whereas clear liquid is healthy.) 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. 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.

Student Name (Giver)
Day 1 Contact (Receiver 1)
Day 2 Contact (Receiver 2)
1
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Disease Lab Questions

  1. How would the results differ if everyone could choose how many contacts to have, including the option to have no contacts? How does this better resemble real life?
  2. How would the results differ if you have only a 20% or 50% chance of contracting the disease after being exposed? How does this relate to the concept of susceptibility?
  3. How would the results differ if people were continuously entering and leaving the group? What if there were subgroups that didn't have much exchange, perhaps because of religious or cultural differences. Why are sick people sometimes quarantined?
  4. Think about modes of transmission. How would the spread of a disease differ if the pathogen is airborne, foodborne, waterborne, requires physical contact like a handshake, or intimate contact like sex, or a kiss? Which would be the most deadly mode of transmission if a terrorist was trying to intentionally create an epidemic?
  5. Are STDs (sexually transmitted diseases) usually spread between strangers or people we know well? Always? Discuss. Why is it important to know your partner's complete sexual history before you decide to have sex? If he/she won't tell you, or if you suspect an active history, consider that you are effectively sleeping with all of his/her former partners. To protect yourself, is it reasonable to propose that you both get tested for STDs before having sex? It's safe to assume that everybody lies about sex!
  6. Why does international air travel increase the risk of a rapidly spreading pandemic? Why are airports, train and bus stations, schools, restaurants, movie theaters, and shopping malls likely locations for a disease to spread? If a disease breaks out in a major city, why is this worse than in a small town?
  7. If a vaccine is in limited supply, why do first responders (police officers, firefighters, paramedics, nurses, doctors) get the first doses? Is this fair?
  8. Many diseases, such as the common cold, don't have visible symptoms during their most infectious stage. Why? What would happen if they did?
  9. Describe a situation where certain people (the old, the young, the immune compromised) are more "at risk" than others to a disease. Why isn't everybody equally susceptible? Why does getting vaccinated protect people who are too young or otherwise unable to get vaccinated?
  10. How would the results differ if the infected person dies very quickly or very slowly after contracting the disease? Which disease will be more evolutionarily successful -- one that kills quickly or one that kills slowly? Why? If you try to "think like a disease" what is your primary objective if you want to be successful? What is the purpose of the host? Explain.
  11. If we expanded this simulation to 4 days, but infected people either die or get better after 2 days, based on a heads/tails coin flip, how much harder might it be to track the course of the epidemic?
  12. What if a vaccine becomes available that prevents infection? Research the concept of "herd immunity" and the percentage of people that need to be vaccinated for it to be effective. Try this measles simulation in your own city or state.

Further Investigation: