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Similarities between the Spread of an Infectious Disease and Population Growth How Does an Infectious Disease Spread? An infectious disease is any disease caused by germs that can be spread from one person to another. Germs include viruses, bacteria and protozoa. What are some infectious diseases? What are some diseases that are not infectious? This activity will simulate the spread of an infectious disease. A simulation is a simplified demonstration of a real biological process. Our simulation will show how an infectious disease can spread from one infected person to other people, who in turn infect others. Instructions 1. Your teacher will give everyone a cup filled with a clear solution. This solution represents your body. Only one person in the class will have a cup that has been “infected”. Obviously, you should not drink from the cup. (In laboratory activities you should never drink or eat anything unless your teacher tells you that it is safe to do so.) 2. In this part of the activity, you will interact with two other students. To interact with another student, pour all of your solution into your partner’s cup. Then have your partner pour all of the mixed solution back into your empty cup. Finally, pour half of the mixed solution back into your partner’s empty cup. Wait for the signal from your teacher, and then move to another part of the classroom and interact with a second student. After you have finished your second interaction, return to your seat. Estimate how many people you think will be infected. 3. Your teacher will come around and put an “infection indicator” in your cup. If you have exchanged solutions with the original infected person or someone else after they became infected, you are now infected and your solution will turn pink. If you have not exchanged solutions with anyone who was infected, your solution will not turn color. 4. Repeat another set of interactions, again beginning with only one student with an infected cup. This time there will be three rounds of interactions. For each interaction, be sure to move to a different part of the room with different students. After the teacher has come around with the indicator, write down how many people were actually infected. 5. Now graph how an infection spreads with increasing numbers of interactions. First, plot a point to indicate that one person was infected before any interactions. How many people would be infected after just one interaction? Next, plot the number of people who were infected after two interactions (from 3 on page 1) and the number of people who were infected after three interactions (from 4 on pages 1-2). Our simulation showed the way a disease could spread if the spread of disease depends on person-to-person contact. Population Growth There are interesting similarities between the spread of infectious diseases and population growth, e.g., the increase in the number of bacteria, plants or animals in a population. For example, in a growing population of bacteria, each bacterium can give rise to two new bacteria after about 30 minutes. This results in a doubling of the number of bacteria every 30 minutes, which is similar to the doubling in the number of infected people after the first and second rounds of disease-spread interactions. Suppose a single bacterium is placed on an agar plate and the number of bacteria in the population doubles every 30 minutes. How long do you think it would take before there would be 1000 bacteria? To calculate how long it would actually take for the single bacterium to multiply to form a colony of 1000 bacteria, fill in the number of bacteria at each time in a table with the following rows: - bacterium at the beginning - bacteria after 30 minutes - bacteria after 1 hour - bacteria after 1 hour and 30 minutes - bacteria after 2 hours - bacteria after 2 hours and 30 minutes - bacteria after 3 hours - bacteria after 3 hours and 30 minutes - bacteria after 4 hours - bacteria after 4 hours and 30 minutes - bacteria after 5 hours How long would it take for the population of bacteria to increase from 1 bacterium to 500 bacteria? How long would it take for the population of bacteria to increase from 500 bacteria to 1000 bacteria? Notice that, when a population doubles every 30 minutes, the number of bacteria in the population increases faster and faster as the population gets larger. This kind of population growth is called exponential growth. To see what exponential growth looks like in a graph, use the data from the above table to plot the number of bacteria on a graph where the x-axis is every hour. If exponential growth continued for 10 hours, the original single bacterium would increase to a population of over one million bacteria. This illustrates how exponential growth can result in very rapid increase of population size.