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Background:
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Infectious diseases are caused by bacteria or viruses. These bacteria or viruses invade a host, multiply in the tissues of the host, cause cell damage in the host, and may be passed on to other hosts.
Bacteria and viruses may enter the body by the respiratory tract, by the alimentary track, by wounds or abrasions in the skin, by sexual contact, or through insect bites. For example, the human tubercle bacillus and the pneumococcus, which infect the lungs, may be expelled by the coughing of one person and inhaled by another or eaten with contaminated food.
There are three primary means of preventing infectious diseases. The amount of infectious bacteria can be reduced by methods such as heat, chemical disinfectants, ultraviolet light, isolation of patients, or simply using a handkerchief when spitting, coughing, or sneezing. Transmission of the infecting agents can be prevented by methods such as purification of water, pasteurization of milk, hygienic handling of food, and killing insect larvae by chemical agents. The resistance of a host to bacteria can sometimes be increased by vaccination or even daily administration of a drug.
Many precautions are taken and much research is done toward controlling the spread of infectious diseases. Nevertheless, scares occur frequently that can almost bring hysteria to a population. In October 1994, for example, ice cream made by Schwan Sales Enterprises in Minnesota was blamed for 400 confirmed cases and 3,000 to 5,000 suspected cases of salmonella food poisoning in up to 35 states.
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Teacher's Notes:
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Take appropriate safety precautions. There is no place for horseplay with medicine droppers and test tubes or with sodium hydroxide.
This is a good motivating activity for students. A considerable proportion of the students "contract the virus" in this experiment. This is a good springboard to start a discussion about AIDS or other sexually transmitted diseases (subjects about which many students seem tired of hearing), or really about transmission of any infectious diseases. The students participate in discussions with new enthusiasm. Comments like "I didn't know it was so easy to get" and "I never knew I had the disease" are common. It is interesting to watch as students try to trace the infection through their population of test tubes.
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Materials:
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Materials are based on a class of 30 students.
30 clean test tubes
28 test tubes already half-filled with distilled water
2 test tubes already half-filled with 0.1 M sodium hydroxide, NaOH
(0.40 g NaOH dissolved and diluted to 100 mL)
30 clean medicine droppers, either glass or plastic
Phenolphthalein solution (purchased or prepared as a 5% solution by mass in ethanol)
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Explanation:
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Phenolphthalein is an acid-base indicator which is colorless in its acid form that dominates up to about pH 9 and red in its base form that dominates above about pH 9. A single medicine dropper of 0.1 M NaOH in any of the test tubes is more than sufficient base to cause the colorless phenolphthalein indicator to turn red (pink in dilute solution).
The minimum number of students who can be infected in this activity is four based on the following reasoning. If the two infected students exchange solutions during the first exchange, no new students are infected. However, during the second exchange, the two infected students must exchange with and infect two more students. In the third exchange, the four infected students can again cross exchange with each other and not exchange with the same person twice. Therefore, only four students total are infected at the end of the activity.
The maximum number of students who can be infected in this activity is 16 based on the following reasoning. The two infected students infect two more in the first exchange. Those four infected students infect four more in the second exchange for a total of eight infected students. Those eight infected students infect eight more in the third exchange for a total of 16 infected students.
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Questions:
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Given that only two students in your class initially had the virus, did you expect more or fewer people to be infected with the virus? Why?
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Given that only two students in your class initially had the virus, determine the minimum number of students in the class that should be infected with the virus. Do this by assuming that in the first exchange the two infected students exchanged solutions with each other and that in the third exchange students originally infected and students infected in the second exchange only exchanged solutions with each other.
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Given that only two students in your class initially had the virus, determine the maximum number of students in the class that should be infected with the virus. Do this by assuming that no infected student exchanged liquid with another infected student in any of the three exchanges.
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Compare your answers in question 2 and 3 to the number that were actually infected. Does the number who were actually infected fall within your range?
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Procedure:
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Instruct each student to take one test tube and one medicine dropper. Inform the students that they are epidemiologists who are interested in the spread of infectious diseases through a population. Tell them that each test tube represents an individual and that the liquid in each test tube represents the "body fluids" of that individual. (To the students all test tubes contain a colorless liquid, look alike, and have a liquid composition unknown to them.) Additionally, tell them that as scientists who are interested in the spread of dangerous infectious diseases, they must take appropriate precautions against infection and must not spill any of the liquid.
Instruct students to exchange one medicine dropper full of liquid with three different students from various parts of the classroom. (Exchange means to withdraw one dropper full of liquid and carefully dispense it into the test tube of another student followed by the other student doing the same to the first student.)
After all students have done this, and returned with their test tubes to their seats, place two drops of phenolphthalien solution into the test tube of each student. If a student has been infected or has "contracted the virus," the liquid in that student's test tube will turn a deep pink color. Have students count carefully the number of students who have been infected. Then inform the students that only two students were infected at the beginning of the activity.
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