Hands-on Activity: Tracking a Virus

Contributed by: Engineering K-PhD Program, Pratt School of Engineering, Duke University

photomicrograph of HIV budding from host cell
A newly made HIV virus is shown in this photomicrograph, immediately after being released from the infected immune cell just below it.
Copyright © NIH


Students simulate the spread of a virus such as HIV through a population by "sharing" (but not drinking) the water in a plastic cup with several classmates. Although invisible, the water in a few of the cups has already be tainted with the "virus" (sodium carbonate). After all the students have shared their liquids, the contents of the cups are tested for the virus with phenolphthalein, a chemical that causes a striking color change in the presence of sodium carbonate. Students then set about trying to determine which of their classmates were the ones originally infected with the virus.

Engineering Connection

Biomedical engineers and pharmacologists continue to work on developing a cure for AIDS. An understanding of the biological mechanisms behind the disease are crucial to develop a vaccine.

Learning Objectives

After students complete this activity, they will be able to describe the type of work that epidemiologists do.

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Materials List

  • 24-32 clear plastic drink cups, 4 to 9 ounce size
  • water, preferably distilled, about 1 liter
  • phenolphthalein solution, about 5 ml
  • sodium carbonate (aka washing soda), 1 scant teaspoon
  • disposable pipette or eye-dropper
  • 24-32 small test tubes
  • test tube rack
  • beaker, flask, or cup
  • permanent marker
  • 24-32 index cards


The associated lesson's Introduction/Motivation provides adequate preparation for this activity. If not, simply announce to the class, "Today you will all share bodily fluids in order to see who gets HIV." That is guaranteed to get their attention!


epidemiology: A branch of medicine that is concerned with the causes, spread and control of diseases in populations.


Set Up

  1. Set the cups out on a table and place the test tubes in the rack. Using a permanent marker, number the cups 1 to 24, 28, or 32, depending on class size. Please read the information about class size in step 3 of the Conducting the Simulation section (below) before you start! Number the test tubes in the same way. It is important that the total number of cups and test tubes used is a multiple of four.
  2. Put about a cup of water into the beaker, and stir in the sodium carbonate (a.k.a. washing soda) until it is all dissolved and the water is clear. Choosing three of the numbered cups at random, pour this solution into them so that each cup is about one-quarter full. Pour some of the remaining solution into three of the test tubes whose numbers correspond with the cups, so that each test tube holds about an inch of solution.
  3. Fill the other cups about one-quarter full with water. Fill the remaining test tubes with about one inch of water. Put the test tubes somewhere out of sight.

Conducting the Simulation

  1. Give each student an index card on which to write his or her name. Have each student obtain a prepared cup, and then write the number that appears on the cup next to his or name on the index card.
  2. Explain to students that they will each "exchange bodily fluids" with three other students, one at a time, following a certain procedure:
  • After choosing a classmate to "share" liquids with, one member of the pair pours all of the water from his or her cup into the partner's cup. Then the partner will pour half of the combined liquids back into the first member's cup. This way the students have mixed their two liquids, but in the end, each has the same amount they started with. Both students should record the number of the cup belonging to the person they just exchanged liquids with. (Demonstrate this exchange process with two extra cups containing tap water, emphasizing the need to record the numbers of the cups.)
  • Each student should then find a different classmate to exchange liquids with, and record the cup number corresponding to this second exchange.
  • Each student should then find a third student to exchange fluids with, and record the cup number corresponding to the third exchange. When all students are finished, each student should have shared liquids with exactly three other students.
  1. Note: This number three is important for the problem solving aspect of the exercise that follows. Thus, it is also important that the total number of participants is a multiple of 4, so classes of 24, 28 or 32 will work perfectly. If you don't have quite enough students to make a multiple of four, it is best if you recruit extra students or adults (including yourself) rather than leave any students out. Any recruits need only be present for the few minutes it takes to do the liquid exchanges. If you have 20 or fewer students, use only two "infected" cups (and test tubes) for the simulation, instead of three.
  2. Have students return to their seats with their index cards and cups of water. Tell the class that, unfortunately, a few of the cups contained "bodily fluids" that were infected with the AIDS virus, HIV, at the start of the simulation. By exchanging bodily fluids with their friends, it is likely that several more students are now "infected with HIV." Then walk around the room, placing a drop or two of phenolphthalein in each cup. Those cups with water that turns bright pink contain the "virus", so each student whose water is pink is now "infected" -- and contagious. Typically, at least two-thirds of the class will have become infected during the exchange process.

Tracking the Virus

Give the students a chance to comment on the results of their "experiment." Probably they will ask who the original "infected" people were, so you should return the question to them, asking, "How can we find out?" Point out that epidemiologists are scientists and medical doctors who try to solve puzzles such as this, and epidemiology is a branch of medicine that is concerned with the causes, spread, and control of diseases in populations.

Expect students to soon realize they need to start by eliminating those students who were not infected at the end of the experiment, and then try to work backwards from there. Treat this as a puzzle for them to solve -- try to keep out of things as much as possible. Give them time to realize that they will need to get organized and devise a systematic way to look at the data they have. They will probably realize they need one or two students at the board to lead the discussion and record information as they go along. Once they get going, it may be easy at first to eliminate students who could not have been the initially infected ones, but it will then become more difficult. They may not be able to deduce the original three infected persons, but they should be able to eliminate all but 4-6 students.

At this point you can tell them that, fortunately, you took a "blood sample" from everyone before they started trading their "bodily fluids." Produce the rack of test tubes, and show how they are numbered to correspond to the cups that were used. Then explain that you can test for the presence of the virus using the same chemical indicator as before. Have a student volunteer put a drop of phenolphthalein in each test tube, and students will then be able to see how close they got to determining the original sources of the virus.

Safety Issues

Phenolphthalein, even in small doses, can act as a laxative. Caution students against drinking any of the liquids used in this activity, and have them wash their hands at the end of the activity. Clean any spills thoroughly.

Troubleshooting Tips

  • It is best to let students choose their own cups of "bodily fluids" at the beginning of the simulation, rather than handing them out. If students choose their own cups, the teacher cannot be accused of intentionally giving a student an "infected" cup. To make distribution of the cups easier, have only three or four students select their cups at a time.
  • When all students have their cups and are ready to begin exchanging fluids, make sure they are on their feet, and encourage them to walk around the room and not just exchange with those right next to them. You can facilitate this by having students who are close friends situated apart from each other in different parts of the room at the onset of the activity. If students only exchange with those in close proximity, or only with their close friends in a clique, the "infection" is more likely to stay in one clump of students and not get distributed throughout the class.

Investigating Questions

  • What percent of the class became infected after exchanging the fluids? What percent of the class was infected originally, before exchanging the fluids?
  • Why is it usually not possible to determine exactly who the originally infected persons were in a situation like this? Usually the sources can be traced back to 4-6 possibilities, but the actual three can only be determined if all the students did their first exchanges simultaneously, then all did their second exchanges simultaneously, and finally completed the third exchanges simultaneously. Instead, during the simulation some students will have already completed their third exchanges before others completed their second. Without knowing exactly who exchanged fluids when, it is nearly impossible to determine who the original three "infected" persons were.


Student assessment is not particularly appropriate at this time, although you can make note of which students participate most actively and thoughtfully in the problem-solving phase of the activity.

Activity Extensions

Students may be interested in reading about other epidemiology case studies and how epidemiologists go about their work. A good start is "Disease Detectives," a National Geographic (June 1991) article that describes physician John Snow's classic work in London to discover the source of a cholera outbreak in the mid-1800s. The article also tells the story of a more contemporary outbreak in west Africa, which is particularly interesting because of the role prevailing cultural practices can play in the transmission of a disease.


Mary R. Hebrank, project writer and consultant


© 2013 by Regents of the University of Colorado; original © 2004 Duke University

Supporting Program

Engineering K-PhD Program, Pratt School of Engineering, Duke University


This content was developed by the MUSIC (Math Understanding through Science Integrated with Curriculum) Program in the Pratt School of Engineering at Duke University under National Science Foundation GK-12 grant no. DGE 0338262. However, these contents do not necessarily represent the policies of the NSF, and you should not assume endorsement by the federal government.

This activity of this curricular unit was originally published, in slightly modified form, by Duke University's Center for Inquiry Based Learning (CIBL). Please visit http://www.biology.duke.edu/cibl/ for information about CIBL and other resources for K-12 science and math teachers.

Last modified: August 23, 2017