Try this: How does an infectious disease spread?

You will need

• Small containers, plastic cups work well (one cup per person)
• Citric acid or lemon juice and water to produce a solution with an acid pH.
• Bromothymol blue pH indicator (fish or pet stores)
• One litre container
• Tap water
• A group of people, no less than eight. This activity works best in a group but you could do it on your own with the same results. You just need to keep track of the cups and the number of interactions you have.

What to do

1. Prepare the infected solution by mixing two heaped dessert spoons of citric acid or lemon juice with 500 millilitres of water.
2. Set out the same number of small cups as people who will be doing the activity.
3. Half fill one cup with the infected solution (clear acid solution) and half fill the remaining cups with tap water. Make sure you don't tell anyone which is which.
4. Give everyone a cup. Before doing so remind them not to drink the solution. This solution represents your bodily fluids.  One person in the group will have a cup that has been “infected.”
5. Now interact with one other person by pouring all of your solution into your partner's cup. Then have your partner pour all of the mixed solution back into your empty cup. Finally, pour half of the mixed solution back into your partner's empty cup. You may need to demonstrate this to the group.
6. After everyone has completed two rounds of exchanging solutions, have them go back to their seats or stand in a circle and predict the number of infected people.
7. Go around the room and drop the Bromothymol blue in the cups; as you're doing so, tell the group that you are putting an infection indicator into their cups. If they exchanged solutions with the original infected person or someone who came into contact with the infected person, they are now infected and their solution will turn yellow. If they have not exchanged solutions with anyone who was infected, their solution will be blue.
8. Have a show of hands to determine the number of people who were infected.
9. Now repeat the process, but have three interactions instead of just two. You need rinse the cups well before starting the process again.
10. If you have enough time and your group is large enough, it may be helpful to have another round with four or more interactions.
11. Dispose of the liquid by rinsing it down the sink with plenty of water or disposing of it the garden.

What's happening?

You should have noticed that the ‘non-infected' water remained blue, the colour of the indicator, because it is neutral. The ‘infected' water (the acidic solution) turns yellow.

In order to predict the spread of the disease, you will notice that the number of infected people doubles after each round of interactions. This doubling in each round results in exponential growth.

As the number of infected people gets larger, it is increasingly likely that one infected person will interact with another infected person, which will slow the rate of increase in the number of infected individuals. This results in a growth curve, with the maximum number of infected people equal to the total number of people in the group.

If you are an avid amateur disease tracker or doing this at school, you could graph how the infection spreads with increasing numbers of interactions. The number of people infected appears on the Y axis and the number of interactions along the X axis.

Applications

This activity shows the way a disease spreads relying on person-to-person contact. Examples of these kinds of disease include pink eye and chickenpox. The spread of the disease in this activity was very rapid. Within a few minutes many people were infected. In real life, infections do not spread as rapidly. Why do you think this is? Use the following websites to explore disease control in our population.

More information

• WHO: Infectious diseases
• How clean are your hands?