Kissing the Epstein-Barr virus goodbye?

Activity 1

A model of disease transmission

This activity models how one person with a disease can spread it to many others.

Materials (for each student)

  • stock solution in a small test tube
  • 1 small test tube
  • 1 Pasteur pipette with bulb

Procedure

Safety notes:

  • Do not allow any solution to come into contact with your skin or clothing.
  • Notify your teacher immediately if a spill occurs.
  • If you splash any solution on yourself, immediately flood the affected area with water.
  1. Transfer half of your stock solution to your clean test tube. This will be your solution for exchanges. Think of the solution as aerosol droplets from a sneeze.
  2. Round 1: Find one other class member at random and exchange one drop of solution. (The exchange is made by using your Pasteur pipette to place one drop of your solution into that person’s test tube and receiving one drop of their solution in return.)
  3. Record the name of your contact in Round 1.
  4. Your teacher will signal Round 2. Find a different contact and exchange one drop of solution.
  5. Record the name of your contact in Round 2.
  6. The teacher will signal Round 3. Find a different contact and exchange one drop of solution.
  7. Record the name of your contact in Round 3.
  8. Test your solution by adding 1 mL phenol red indicator to your test tube and record the colour. ‘Infected’ solutions are red; all others are yellow.
  9. Rinse out all your glassware thoroughly.
  10. On the blackboard, record your name, your contacts and whether your solution was ‘infected’ or not.
  11. Using the class records on the blackboard, try to trace the transmission route through the class. Attempt to determine the original disease carrier. (You may only be able to narrow it down to two or three individuals.)
  12. To establish the original disease carrier, test the stock solutions of the possible disease carriers for the red colour indicating ‘infected’.

Questions

  1. What is the maximum number of ‘infected’ individuals possible after three rounds?
  2. Why might the observed number be lower than the maximum?
  3. What method of disease transmission is not simulated by this model?
  4. Suppose that instead of one exchange of solutions each round, you exchanged as many times as you wanted during a specified time period.

    • What differences might be seen in the outcome?
    • How would this change in rules affect your ability to trace the transmission routes?
  5. What do public health officials do to help control the spread of infectious diseases?

Teachers notes

Background information

Phenol red is an indicator solution that turns pink or red in alkalis. The ‘infected’ stock solution is 0.1 M sodium hydroxide (NaOH). This concentration should produce a pH that will ensure that even the diluted ‘infected’ solutions will turn red in phenol red. All of the ‘uninfected’ solutions should be yellow in the presence of phenol red. Because the pH of tap water varies, dilute (0.001 M) hydrochloric acid (HCl) is used as the ‘uninfected’ stock solution to ensure ‘uninfected’ samples turn yellow in phenol red. Also ensure that the test tubes do not have residual traces of acids or alkalis.

Preparation

This activity depends on one student in the class having an 'infected' solution that will turn red when phenol red is added. Set up test tubes to be collected so that one tube contains 0.1 M NaOH and all the others contain 0.001 M HCl.

    Reagents

  • 0.001 M HCl: (CAUTION. ALWAYS ADD ACID TO WATER.) Prepare 1 M HCl by adding 11 mL concentrated (32.3%) HCl to 89 mL water. Prepare 0.001 M HCl by adding 1 mL of 1 M HCl to 99 mL of water. (Prepare enough 0.001 M HCl to give every student (minus one) 2 mL.)

  • 0.1 M NaOH: 0.4 grams of NaOH made up to 100 mL with water. (Prepare enough 0.01 M NaOH give 2 mL to at least one student.)

  • Phenol red solution: 0.1 grams phenol red dissolved in 100 mL water. (Prepare enough phenol red solution to give each student 2 mL.) Test both stock solutions and diluted samples of the two stock solutions with the phenol red before use.

    If you use the phenol red to test each of their stock-solutions at the end of the third round, you will be able to help students assess any ambiguous results. Use a clean 1 mL pipette to add the phenol red to the students’ test tubes.

Answers to questions

  1. A maximum of eight people could be ‘infected’ at the end of three rounds.
  2. The number would be lower than the maximum if two ‘infected’ people chose to exchange samples with the same person.
  3. Examples of disease transmission not simulated by this model:

    • one infected person sneezing into a room full of people;
    • a single tap infecting a community with cholera.
  4. With an unlimited exchange of samples you would expect:

    • there would be many more people ‘infected’ at the end of the round;
    • it would be much more difficult to trace the ‘infection’ route because there would be many more instances of multiple ‘infections’.
  5. Public health officials attempt to control infectious diseases by minimising the number of contacts infected people make with the rest of the community by encouraging infected people to stay home. If the disease is serious, infected people are put into isolation wards.

Notes

Make sure that students understand the procedure before beginning.

It is possible to make a number of variations to this activity:

  • start with more than one ‘infected’ person in the class;
  • allow more than one exchange each round;
  • don’t have students record the names of their contacts in each round;
  • have only a one-way transfer rather than a two-way exchange.

The first three variations would give a more complicated pattern to decipher while the fourth suggestion would make the transmission route easier to determine. The first three variations would represent more closely a real-life situation, and give a better idea of the difficulties faced by epidemiologists.

External sites are not endorsed by the Australian Academy of Science.
Posted November 1997.