Sourcebook Activity 6.3.3 Deducing the sources of communicable diseases (biology)

Epidemiologists study the origin and spread of diseases. Communicable diseases are passed from one person to another as pathogens travel through air, fluids, or direct contact between individuals. Table 6.9 lists the leading cause of death in developed and developing nations. The underlined items are communicable diseases. Note that the third, fourth and eighth leading causes of death in developing nations are communicable disease, while none of the top ten causes of death in developed nations are in this category. This is an indication that epidemiologists, physicians, and public health specialists have done a good job of identifying the sources of disease and the ways to control them.

Epidemiologists often use survey data and deductive reasoning to determine the sources of disease. In this activity you will collect your own data (or use the data in table 6.10 if you are unable to do the activity) in an effort to determine the source of a “disease”.

Each person is provided with two test tubes that are placed upright in clear plastic cups. Each pair of test tubes is marked with an identifying number and labeled C (for control) or E for experimental). For example, student number 5 has two identical test tubes labeled 5C and 5E. All test tubes contain an equal amount of water with the exception of one pair that contains an equal volume of dilute solution of sodium hydroxide (0.01 N; made by adding 0.4 grams of sodium hydroxide to a liter of water). Avoid spilling fluids, and rinse with water if you contact the fluid.

The test tubes containing dilute sodium hydroxide represent a person with a communicable disease. All other tubes represent healthy people. No one except the teacher knows who received the “infected” tubes, and it is the goal of the class to find out. At the teacher’s command, pair up with another member of the class, record their number, and exchange a couple milliliters of fluid with each other using a Beral pipette or eyedropper. Alternatively, you pour a little from your container to theirs and visa versa. This is time-1 (T1). Repeat this process twice when instructed by the teacher (T2, T3). After the third (for classes of 20 or less) or fourth (for classes of 40 or less) exchange, the teacher will place a drop of phenolphthalein solution in all of the experimental test tubes. A pink color indicates “infection” because phenolphthalein turns pink in basic solutions such as the one containing sodium hydroxide, and all other cups contaminated by it. Create a table on the board or an overhead transparency and indicate the people who exchanged at each interval as shown in table 6.10. In the final column, circle the people who are “infected” as indicated by pink coloration. Using the following premises, deduce who the possible sources of the disease are. You should be able to narrow to two possibilities.

  • premise 1- A clear tube has not mixed with any “infected” tubes.
  • premise 2 - A pink tube has mixed with “infected” tubes (or is the source).

Using logic, one can identify those tubes that could not have been the source. One can deduce that if a test tube is clear after the final exchange, then none of the tubes that it mixed with could have been the source. For example, in table 6.10 test tube 3 is clear, and therefore one can deduce that tubes 14, 19 and 20 could not have been the source. Tube 2 is infected, but cannot be the source because it exchanged with tube 20, which is still clear at the end. Tube 20 would have been pink had tube 2 have been the source. Using deductive reasoning, identify the two possible sources of the disease.

Once all of the students have predicted the source of the disease, the teacher will place an equal amount of phenolphthalein in each of the control tubes. Only the tube representing the source of the disease will turn pink. The control tubes represent abstinence (in the case of sexually transmitted diseases like HIV/AIDS or syphilis), vaccination (in the case of measles of tetanus), or masks (in the case of respiratory illnesses).