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        Epidemiologists: Disease Detectives

        In this lesson, students learn about epidemiology and the role epidemiologists play in responding to disease outbreaks and protecting public health.

        Lesson Summary


        When disease spreads within a community, public health workers try to understand what is happening. They interview people, study medical records, and—just like detectives—try to make sense of the clues they gather. Then they can inform the public of ways to protect themselves from exposure. Epidemiology is widely considered to be the basic science of public health. Epidemiologists count cases of disease (or injury), consider the distribution of the cases, and define the affected population. If a problem is identified, they use data they collect to try to determine its cause and how it is being transmitted. They also recommend how best to control its spread within the population.

        This lesson uses a real-life case to demonstrate how epidemiologists and other public health workers respond when there is a suspected outbreak—or sudden increase in the number of cases—of a disease. The lesson begins with a video that recounts the early days of epidemiology and its pioneering scientist, John Snow. After a brief review of what is meant by public health, students watch a video in two parts that documents an actual outbreak of West Nile virus in New York City, and then learn how epidemiologists ultimately helped solve this public health mystery. Students also learn through class discussion the various means by which infectious diseases are transmitted and that environmental factors may help or hinder their spread. A final activity gives them a chance to extend the outbreak investigation they saw in the West Nile virus videos. They apply what they’ve learned to interpret maps showing the spread of the disease across the country and in their own state.


        • Understand the various methods of disease transmission
        • Identify the process epidemiologists use to determine the cause of a disease outbreak
        • Apply investigative epidemiological skills to interpret data and help protect the public health

        Grade Level: 9–12

        Suggested Time

        • One to two class periods

        Media Resources


        Before the Lesson

        • If possible, arrange computer access for all students to work in pairs, to encourage more conversation.

        The Lesson

        Part I: A Public Health Mystery

        1. Show students the John Snow: Pioneer of Epidemiology Video. This dramatization recounts the beginnings of epidemiology and how one doctor developed a systematic process employing logic, statistics, and mapping to determine the cause of a cholera epidemic that killed hundreds of people in London in the summer of 1854.

        2. Ask the class to define what the term public health means. Then ask them how they would get information about events, behaviors, or environmental conditions that affect the public health. Explain to the class that today a number of different parties play a role in safeguarding the public health, including medical practitioners (e.g., doctors, nurses), administrators, investigators, laboratory workers, and, as shown in the following video, veterinarians and wildlife pathologists. (Pathologists are specialists who study the nature of diseases, especially the structural and functional changes produced by them.)

        3. Show students the Mystery Illness in New York City Video. It describes the beginning of a more recent disease outbreak whose unknown origin threatened to cause panic throughout the city of New York. It also highlights the first few steps of a public health investigation. You may point out to students that a public health investigation starts with a question. For John Snow it was, How do people become sick with cholera? In the case that follows, the question is, Why are the crows dying? Investigators then decide what data to collect, analyze the collected data, make claims based on the evidence they find, and provide explanations. You may also reinforce the idea that, as is often the case in science, investigations can be complicated and steps may not follow a prescribed order.

        After showing the video, ask the class:

        • Why do you think mosquito breeding could be relevant to the investigation? Do you know of any diseases spread by mosquitoes? What environmental factors might affect mosquito populations?
        • What did investigators know—and not know—about the disease outbreak to this point? In other words, what data did they have to work with?

        4. Focus on epidemiology and the role of an epidemiologist. First, review definitions. (Epidemiology is the study of diseases as they occur in living things. It focuses on understanding the transmission, spread, and control of diseases. An epidemiologist, like Annie Fine, the New York City public health worker who appears in the video, is a scientist who investigates the factors that cause disease.) Then ask students to relate what they saw in the video to the process epidemiologists use in their work: epidemiologists go into outbreak areas, interview people, and gather data. This helps them either form a hypothesis that may explain the outbreak, or prove or disprove a hypothesis they already have. To organize the discussion, you may ask:

        • Whom did New York City public health workers interview when they began their investigation? What questions did they ask?
        • What did they suspect might be responsible for transmitting disease to the human population? How did they come up with this idea?
        • What did the investigators do next to test their hypothesis?

        Part II: Detectives at Work

        5. Ask students to name some of the ways in which infectious diseases are transmitted from one organism to another. The primary means of transmission are direct or indirect contact; airborne transmission; food, water, or blood contamination; and vector-borne transmission. Vector-borne transmission occurs when an infected organism—such as a parasite, insect, or animal—transmits the disease to another host, such as a person or animal.

        Lead a brief class discussion about how understanding the source of transmission, or how a disease is passed on, can help stop its spread. Have students consider some of the factors that may bring diseases to a given place (e.g., human or animal migration, vacation or business travel, climate change, natural disasters).

        6. Show students the Mystery Solved! West Nile Virus Video, in which investigators ultimately determine the source of the virus’ transmission.

        7. After watching the video, ask students what evidence led investigators to conclude a mosquito was responsible for transmitting the disease. Have them discuss how the CDC made its initial diagnosis—St. Louis encephalitis—and why not everyone was convinced the initial diagnosis was correct. What, in the end, convinced the CDC to reconsider its diagnosis?

        Part III: Activity

        8. Now that students know where West Nile virus was first discovered, how it spreads, and its potential health consequences, have them step into the shoes of an epidemiologist. Split the class into two groups. One will look at a national data set, and the other will look at a state-by-state data set. Each data set contains information about human cases of West Nile virus reported since the emergence of the disease in the United States in 1999.

        Students working with the national data sets should prepare answers to these questions, using the referenced reports, as needed:

        • What observations can you make about the spread of this disease across the country, beginning in 1999 and through 2009? Note the appearance of any trends. Use the report: West Nile Activity in the United States - Maps Document.
        • What might explain any increase or decrease in reported cases?
        • What are the most common symptoms among human patients? Use the report: West Nile Activity in the United States - Human Case Count Document.
        • What is the incidence of each of these symptoms (i.e., how often did it happen), and what does this tell you about the severity of each symptom?

        Students working with the state data set should be able to answer the following questions based on information in the maps found here: CDC: West Nile Virus ArboNET Map:

        • How has West Nile virus affected your state? Graph the number of human cases reported year to year. Do the data support any trends concerning incidence, or the rate or range of occurrence?
        • How are dead birds indicators of West Nile virus? Have dead birds been reported in your state?
        • Looking at a sampling of states, during which time of year is this disease most prevalent? How does this relate to what you know about mosquito activity? What environmental factors might affect mosquito activity?
        • Optional: Learn how your state’s public health department is educating the public about lowering the risks of disease. Locate its Web site (in a search engine, type in your state name and “health department”), then search within the site for West Nile virus.

        Check for Understanding

        After the two groups have analyzed the data, have each group report to the class as part of a simulated press conference to notify the public that a case of West Nile virus that has been confirmed in your area. Have the group that worked with the national data set provide a general summary of West Nile virus cases in the United States, including symptoms to watch out for; have the group that worked with the state data set discuss local incidence as it pertains to the state they live in. For example, students could make maps that show incidence by county over the past 3 to 5 years or devise a different “visual” to help represent the data they want to explain to the public.


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