Purpose: Students will practice making predictions and drawing conclusions. The activity will help students understand how infections spread, especially among organisms living in close proximity.

Procedural overview: In this activity, students will simulate the transmission of an infectious disease through a population. Students will research one infectious disease in plants or animals (white-nose syndrome in bats is used as an example) and draw connections between that disease and the simulated transmission.

Approximate class time: One class period to complete the activity questions, activity and analysis. One class period for research.

Supplies:

Test tubes (one for each student)
Test tube racks (enough to display all the test tubes)
Pipettes with bulbs or droppers
Safety goggles
Nitrile gloves
1 M Silver nitrate
1 M Sodium chloride
Water
Internet access or materials for background research on an infectious disease of your or your students’ choice

Directions for teachers:

In this activity, students will research an infectious disease in a plant or animal population (white-nose syndrome in bats is used as an example) and then perform an experiment to show how an infected individual can infect the rest of the population. As potentially harmful chemicals will be present, it is important for students and the teacher to wear safety goggles and gloves at all times during the experiment.

Before starting the activity, students should search the Science News or Science News for Students archive for an article describing the spread of an infectious disease within a plant or animal population. They can use that article as a starting point for additional research. This can be done with a partner in class or as homework.

Alternatively, you can provide students with the research materials for an infectious disease of your choice. If you choose to use white-nose syndrome, the Science News for Students article “New treatment offers hope for bats battling white nose syndrome” and Figure 1 of the primary research study “Environment, host, and fungal traits predict continental-scale white-nose syndrome in bats” would be a good starting place. You can send the students to computers, use a projector to display the article or distribute printouts depending on your classroom needs. After their research, students should answer the questions provided.

Before the simulation begins, the teacher should number test tubes so there are enough for each student in the class to have one. Fill all but one of the test tubes half full with water, and fill the last test tube half full with the sodium chloride solution. Record which test tube contains the saltwater and save this information for later. Place all of the test tubes in racks at the front of the room, so all students may select one test tube at random after they have completed their research and answered the accompanying questions.

Then, explain to students that all of the test tubes in the racks contain plain water except one, which has a solution of sodium chloride and water (aka saltwater), and that the saltwater will form a precipitate in the presence of silver nitrate. Each student will obtain one of the test tubes, and together they will perform a group experiment in three rounds. During each round, each student will exchange half of the liquid in his or her tube with half of the liquid from one other student in the class. Then, the students will find new partners for the next round. The students should record the number from each test tube with which they exchange their liquid.

After the third round of exchanges, the students will replace their test tubes in the racks at the front of the room. The teacher will place a couple of drops of silver nitrate into each of the test tubes and inform the class which test tubes form a precipitate. In the table provided, each student will record whether his or her test tube formed a precipitate and which of the test tubes he or she exchanged liquids with formed a precipitate.

After the simulated transmission, students will answer questions about the simulation and draw connections to the infection they researched.

Directions for students:

Disease research

Follow your teacher’s instructions to do background research on an infectious disease in plants or animals. Think about what causes the disease, how it is spread and how it affects the organisms it infects. Use your research to answer the following questions:

1. What causes the infectious disease you researched?

Student answers may vary, but those who researched white-nose syndrome may describe that white-nose syndrome is caused by a fungus called Pseudogymnoascus destructans that invades the skin of hibernating bats, including their wings. It causes bats to wake up more frequently during winter hibernation, using up their limited fat reserves very rapidly.

2. What environmental conditions are needed for the disease you researched to thrive?

Student answers will vary as each infectious disease has its own set of environmental conditions under which it thrives. Many of these microbes are temperature sensitive, so extreme heat is not suitable. Additionally, areas that are too cold may prevent the infections from thriving. For white-nose syndrome, hibernating bats lower their body temperatures and the caves where the bats hibernate are cold and humid, an environment where the fungus thrives.

3. How is the disease you researched spread?

Student answers will vary, but most infectious diseases are spread through direct contact with an organism that currently has the disease, or through some form of external transmission, such as through the air or through water. For white-nose syndrome, when bats hibernate for the winter, they are living in close quarters for a long time. Just as humans who stay inside together at school or work during the winter often share colds, bats that are close to each other can share white-nose syndrome.

4. What are the symptoms of the infectious disease you researched?

Student answers will vary, but those who investigated white-nose syndrome should discuss the white growths that appear on the snout of the animals. These growths tend to spread across the bat’s face.

5. Is there a cure for the infectious disease you researched?

Student answers will vary, but those who investigated white-nose syndrome should suggest that scientists have tried antifungal and antibacterial methods to cure the disease, but with little success. How a disease is treated depends on what causes it. If it is bacterial, then antibiotics could help kill it. If it is viral, then there are ways to treat the symptoms, but usually no way to eliminate the virus.

Disease spread

When your teacher instructs you to do so, put on your safety goggles and gloves and collect one of the test tubes provided; record your test tube number in the table below. One of the test tubes contains a solution of sodium chloride and water (aka saltwater) and will form a precipitate when a reagent, silver nitrate, is added. The rest contain water. During each round of the experiment, you will find a student in the class who has not yet been your partner and using your pipette or dropper exchange half of the liquid in your test tube for half of the liquid in his or her test tube. Repeat this process for three rounds. After the third round of exchanges, place your test tube in the rack at the front of the room. The teacher will then add silver nitrate to each tube and inform the class which test tubes form a precipitate.

Record in the table provided the number of your test tube, the test tubes you exchanged liquid with, and which test tubes formed precipitates at the end of the experiment.

	Test tube number	Did the test tube form a precipitate?
My test tube
Round 1 Exchanged with:
Round 2 Exchanged with:
Round 3 Exchanged with:
All test tubes that formed precipitates:

Compare the data you collected with other students in the class. See if you can determine which test tube was the original one that held the saltwater.

Data analysis

6. At the end of the first round of the experiment, how many test tubes have saltwater?

There are two test tubes with saltwater after Round 1: the original test tube and the one that tube exchanged liquid with.

7. At the end of the second round of the experiment, how many test tubes contain saltwater?

There are four test tubes with saltwater after Round 2: the original test tube and the one that tube exchanged liquid with during Round 1, plus the tubes each exchanged with during Round 2.

8. What is the maximum number of test tubes that could contain saltwater at the end of Round 3 of the experiment? What percentage of the class would this be?

The maximum number would be eight: the four tubes with saltwater at the end of Round 2, plus the four tubes those four exchanged liquid with during Round 3. This is (8/[total student body] * 100) percent of the class.

9. What equation can model the maximum rate at which the test tubes can become contaminated with saltwater, if everyone exchanges n times?

maximum rate of test tube contamination = 2ⁿ

10. How many of the test tubes in your class contained saltwater by the end of the activity? What percentage of the class’s test tubes is this?

Student answers will vary depending on with which tubes exchanged liquid; students should record up to eight test tubes and up to 100 percent.

11. Why might the number of test tubes containing saltwater in your classroom be different than the maximum number you calculated in question eight?

In later rounds of the experiment, test tubes with saltwater can exchange liquid with other test tubes with saltwater, rather than with tubes with regular water. This would reduce the number of tubes that saltwater would spread to.

12. How could you determine which test tube was the one that originally contained saltwater?

By mapping out which test tubes exchanged liquids at each step and which test tubes formed the precipitate at the end. Any test tube that did not have saltwater at the end must have only exchanged with freshwater test tubes for every round. For instance, if Test Tube No. 1 has freshwater at the end, that means that every test tube it exchanged with in any given round must have had freshwater in that round, even if it ended up with saltwater in the end. Mapping out which test tubes had freshwater at each step in this manner would help narrow down the possible original saltwater test tube.  

13. How does this activity relate to the spread of the infectious disease you researched?

The activity showed how just one infected organism can pass the disease across the population through some form of contact. As the infected individuals come into contact with others, they are likely to spread the disease. The more contact there is between individuals, the greater the infection rate. With white-nose syndrome, the bats live and hibernate together in large colonies. This means the rate of infection is quite high.

14. Based on the research you have done, what factor(s) could affect the spread of an infectious disease?

Many infectious agents are temperature sensitive, so areas that are warmer or colder than the pathogen’s usual habitat may limit its survival and thus its spread. Additionally, living things usually have some form of defense against invading pathogens. Mammals, like bats, have an immune system that attempts to fight off the disease. Plants also produce chemicals that can impact a pathogen’s ability to infect its host.  

15. What is the likelihood of surviving or dying from the infectious disease you researched?

Answers will vary depending on the disease, but the survival rate of bats with white-nose syndrome can be between just 0 and 10 percent of the population once infected.

Bonus questions

16. Climate change is increasing global temperatures, both on land and in the oceans. How might climate change affect the future spread of the infectious disease you researched?

Increased temperatures might be causing various fungi and other organisms to adapt to higher temperatures. Many of these infectious organisms have been temperature sensitive, meaning they do not survive at higher temperatures. As the planet warms, and the infectious organisms adapt, the range of heat tolerance may also increase. The carriers for diseases may also change their ranges. For example, a disease like malaria that is currently limited to the hot, humid areas of the tropics could make its way into higher latitudes as the ranges of the mosquitoes that carry it expand. Other factors like changing patterns of drought and flooding might also affect the spread of infectious diseases.

17. What can science do to help prevent the spread of infectious diseases?

Answers will vary but might include a discussion of working to slow, decrease or reverse climate change. Students should also discuss the development of vaccines to prevent the spread of diseases or treatments to prevent the growth of infectious agents, including those based on genetic research. For white-nose syndrome, this might include research into using bacteria to help treat the disease.