Phenolphthalein indicator solution (see resources below).
Substitution for Phenolphthalein indicator: Water from boiled red (purple) cabbage.
To make the cabbage solution: Boil a half of a head of red (purple) cabbage in a large pot of distilled water. Using tap water will affect the pH of the solution and the results of the activity. Strain out the cabbage and keep the water. The water should turn purple. Refrigerate any portion that you do not use immediately. This can be kept refrigerated for a day or two. Frozen extract can be kept for months. WARNING: This is a very good indicator and it is inexpensive, but it has a strong smell.
Each student will need a numbered clear plastic cup. Fill all of the cups about half full with distilled water except for one. Mix a saturated solution of baking soda and water by adding baking soda to the water, then stirring or shaking vigorously. Repeat until no more baking soda will dissolve into the solution. Let this saturated solution stand until it is clear and then pour the clear baking soda solution into the final cup to the same level as the other cups. Be sure to note beforehand which numbered cup has the baking soda solution. This baking soda cup is the “infected” cup.
If you teach multiple classes, be sure to use a different number for each class!
Give each student a cup. One student will receive the cup with the “infected” solution – you should know which one this is, but they should not.
Select which students are to be vaccinated and distinguish these students from the rest by giving them a sticker to wear. Explain to these students that the sticker represents a “vaccination” against a common infectious agent currently going around. These students will have vinegar poured into their cups at the end of the activity (see below). Do NOT choose the person with the “infected” cup for this part.
Tell all of the students that they will have three minutes to visit with other students. Each visit will have two steps. First, tell each other their names. Second, share their liquids. To share liquids, one of them will pour all of their liquid into the other’s cup. Then they pour half of the liquid from the filled cup back into the first cup. Point out that this is a model for how infectious agents can spread. Sharing the water could represent direct contact, touching something an infected person or animal has touched, sneezing on someone, transfer of bodily fluids, etc.
It would be good to have two students role play this with plain water to demonstrate the procedure. For this scenario you could tell the students they were modeling cattle or another animal. You could brainstorm ideas for what the contact type could be, like cattle touching noses, coughing, licking a surface of a feed tub….
Give them about three minutes to visit with up to five students depending on your class size. You can decide how many. Make sure they all do the same number.
At the end of three minutes, assemble the students in a line. Explain to them that, at the start of the exercise, all students except one had cups filled with water only. One student, however, had a cup filled with a saturated solution of baking soda. This cup represented the person or animal that was infected. Also, tell them that baking soda is a base. (You may need to clarify or at least ask to be sure they understand the concept of acids and bases.)
Before determining which students are infected, have the students that were vaccinated step forward with their cups. Pour a small amount of vinegar (approximately 20 mL) into the cups of these students representing the vaccination. You may want to test this ahead of time to determine how much vinegar that you will need. Explain to the students that vaccinations only work if received prior to being exposed to an infectious agent but that, for the sake of this demonstration, the “vaccination” or vinegar solution had to be added at the end. Make sure that the students understand that receiving a sticker before visitation is what prevented them from becoming infected.
Now explain to all of the students (and show them) that you have an indicator solution, phenolphthalein or cabbage water. Ask if anyone knows what an indicator solution does. (It changes color to indicate whether something is an acid or a base.) Tell them that phenolphthalein (or cabbage water, if used) acts as an indicator for bases. In an acid solution or a neutral solution, it is colorless. (Show students the bottle.) But in the presence of a base, it turns pink. The color that the cabbage juice indicator will turn will depend on its strength. You should test it ahead of time to determine the colors.
Go down the line and put a few drops of phenolphthalein (or cabbage water) into each student’s cup. If the student is “infected” (i.e. if the contents of his or her cup turn pink) have the student move to a designated part of the room.
Once everyone has been tested, write on the board the number of “infected” people. Be sure to observe which student had the original “infected” cup. You may wish to then dispose of cups and liquids before going any further.
You could see if the class can determine the originally infected person. You can then share who the “infected” person was.
Class Discussion: Discuss some things with the students to make sure they understood the activity:
Begin by going over the numbers – total number of students, number of infected students at the start, total number now infected.
Discuss how this model accurately reflects many real-world situations.
Discuss how this model differs from some real-world situations. (If you get a cold, does everyone in your family also get a cold every time? Why not? Also review that vaccinations are given prior to exposure to an infectious disease and not after.)
Point out how this kind of disease transmission is much more of a problem in species where you have a large number of animals in a given area, like a herd of cattle, bat cave, or large numbers of dogs in a kennel. For example, rabies is rare in bears because they are such solitary animals.
Have the students discuss how vaccination prevented this type of transmission. Make sure that the students understand that humans or animals must receive vaccinations prior to being exposed to a virus or bacteria to be protected. Also, ask them what happens to one’s overall health when a person or animal is exposed to some kind of disease. Does being less healthy before exposure have anything to do with how the body deals with the disease?
Have students complete the Spread of Infectious Disease Worksheet
The disease transmission activity can model the transmission of a vector-borne disease more specifically by assigning the role of “vectors” (here in the form of mosquitoes) to some of the students in the class.
Additional materials needed: Syringes (at least 10mL, one for each mosquito) with NO needles.
Fill all cups with distilled water. It is recommended that somewhere between 70-80% of your students represent people, but you can experiment with different set-ups as you like. Determine how many of the mosquitoes will be infected. Fill the uninfected mosquitoes’ syringes with distilled water and the infected mosquitoes’ syringes with the baking soda solution.
Explain that ___ number of mosquitoes, some carrying Yellow Fever or perhaps West Nile virus, have just entered the classroom.
Identify the “mosquitoes” with name tags, stickers, hats, or some sort of costume, and explain how the mosquitoes bite people: A “mosquito” bites a person by dumping the contents of its syringe into the contents of the person’s cup and then refills its syringe from that person’s cup. Each mosquito will bite four people.
Have the students PREDICT how many people will be infected at the end of the activity. RECORD this prediction.
Again have students visit, but this time without sharing liquids. Have each “mosquito” go around the room and bite four people.
Test to see who is now infected by putting a few drops of phenolphthalein (or cabbage water) into each cup. Ask the students why we did not test to see which mosquitoes are infected (the correct answer is because there is no easy way to see if a mosquito is carrying Yellow Fever or West Nile Virus, since they do not show symptoms like people or animals).
Record the number of infected people. Compare the students’ PREDICTIONS to the ACTUAL DATA.
Discuss how this activity is like the spread of a real disease.
Discuss any other topics like if the mosquitoes had favorites and if real mosquitoes favor certain hosts.