Emerging & Re-emerging Infectiious Diseases
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Activity 1

3. Explain that the quiz emphasized the impact of infectious diseases on people's health and well-being. Point out that even though medical advances in the last century have resulted in far fewer deaths from infectious diseases than at any other time in history, those diseases are still the leading cause of death worldwide and the third leading cause of death in the United States. Explain that in this activity they will learn about some infectious diseases that cause problems in the world today.

You may need to distinguish infectious diseases from noninfectious diseases. Ask students to review the Causes of Death Quiz and identify some of the infectious and noninfectious diseases listed there. If necessary, point out that noninfectious diseases such as cancer, heart disease, and cystic fibrosis cannot be "caught," and that infectious diseases such as AIDS and tuberculosis are caused by living (or quasi-living, in the case of viruses and prions) agents that can be transmitted from one individual to another.

Identifying a disease as "infectious" or "noninfectious" has recently become more complex than it used to be. Researchers have discovered that infectious agents may play a role in some diseases that were previously considered noninfectious, chronic conditions. For example, there is evidence that gastric ulcers are caused by Helicobacter pylori bacteria. Similarly, infection by Chlamydia pneumoniae may contribute to the development of cardiovascular disease, leading some people to question whether heart disease might be infectious.

assessmentCirculate among the teams while they categorize their diseases in Steps 5, 8, and 10 for an informal assessment of students' skills in organizing information.

4. Organize students in teams of three and distribute five Disease Cards made from Master 1.2 to each team.

Distribute the cards in such a way that each disease is reviewed by at least one team.

5. Explain that scientists find it useful to group diseases in different ways, depending on the problems they want to address. As an example, display the first classification criterion on Master 1.3, Disease Classifications, and direct the teams to review their disease cards and sort them into piles that represent different types of infectious agents.

An important science process skill is identifying commonalities and differences and devising classification systems. In this step, students have the opportunity to practice this skill, and in Steps 7, 9, and 10 they consider the usefulness of classifying diseases in various ways.

6. Solicit titles for the categories identified from several teams and write them on the appropriate place on Disease Classifications. Then, ask the other teams to name one or more diseases they classified in the categories and write these into the appropriate columns. Ask students to describe the symptoms of each disease as they do so.

Möbius StripThe discussion in Steps 7 and 9 are opportunities to point out the contribution of basic research to the development of effective treatments and preventive measures for many diseases. For example, research on the life cycle of Schistosoma identified snails as an intermediate host, revealing an important point for preventive measures. Scientists also recently discovered a drug that kills adult schistosomes, reducing the possibility of severe liver disease and interrupting the organism's reproductive cycle. Continuing research likely will lead to effective treatment and preventive measures in the future for diseases like AIDS that are currently incurable.

7. Ask students to suggest reasons why scientists might find it useful to classify diseases based on the type of infectious agent.

If students need help with this, ask them to review the treatment for each of the diseases within a category and the evidence (symptoms) that occur in each. Students should notice that diseases caused by the same type of infectious agent tend to have similar types of treatment strategies, and that similar symptoms occur in diseases caused by different types of agents. It is useful to classify diseases by the type of infectious agent because that indicates the type of treatment that may be effective better than does a review of symptoms.

8. Reveal the next classification criterion on Disease Classifications and ask students to re-sort their disease cards based on this criterion (the mechanism of transmission for each disease).

9. Repeat Steps 6 and 7 for this criterion.

It is useful to classify diseases by the way they are transmitted because a disease's mode of transmission may suggest an effective preventive measure. For example, the spread of diseases such as AIDS and Ebola hemorrhagic fever that are transmitted by intimate contact can be stopped or reduced through education and elimination of some behaviors (such as burial practices in which family members disembowel the deceased in nonsterile conditions) and institution of other behaviors (such as proper disease control measures in hospitals). The spread of vector-borne diseases such as malaria can be prevented by measures that reduce the size of the vector population or that limit contact between humans and the vector.

10. Reveal the last classification criterion, history of the occurrence of the disease, and repeat Steps 5, 6, and 7.

Students likely will identify two categories: "new" (for example, AIDS, Ebola, and Legionnaire disease) and "old" (for example, strep throat, guinea worm disease, pneumonia, polio, and tuberculosis).

If this is the case, fill these headings into the first two columns on Disease Classifications and list the diseases named by students. Then challenge them to re-examine the "old" diseases they listed and to sub-divide that category. Assist them by asking a question such as, "Is there any difference in the history of the 'old' disease tuberculosis and the 'old' disease pneumonia?" When students make the appropriate distinction, add the new headings for the second and third columns on Disease Classifications and relist the diseases accordingly.

Students should note that whereas all of the old diseases are described as "present from antiquity," the incidence of some of them has increased recently (in particular, the incidence of some has increased recently after declining in the past). The two categories from the subdivided "old" category could be renamed "Old and Increasing" and "Old and Remaining Constant."

puzzle pieceThis step focuses students' attention on the major concept of this activity and the module: Infectious diseases are an increasing health concern in part due to emerging and re-emerging diseases.

11. Supply the labels "Emerging" for the apparently new diseases, "Re-emerging" for diseases that have recently increased in incidence after a decline, and "Endemic" for diseases that have remained relatively constant in incidence. Write these labels at the heads of the appropriate columns.

The disease cards provide examples of all three types of diseases, as shown in Figure 17.

Both polio and guinea worm disease are diseases that have declined dramatically and, hopefully, are on their way to global eradication. Cholera and influenza are more complicated examples that are less easily classified. Based on the information on their cards, students will likely classify cholera as a re-emerging disease and influenza as an endemic disease. Depending on the sophistication of your students and the time available, you may simply accept their initial categorization or you may choose to share the additional information below and ask them where they would categorize these two diseases. In either case, note that the categorization of infectious diseases into these three areas is somewhat subjective, and different researchers may categorize them differently based on the weight they give to various characteristics.

Cholera may be classified as either re-emerging because of increasing incidence due to the spread of the disease to Africa, or emerging because of the appearance of the new strain Vibrio cholerae 0139. This strain combines the greater virulence of the classic V. cholerae strain with the long-term survivability of the V. cholerae strain called El Tor.

Influenza is probably most accurately classified as an emerging disease because, although the flu occurs every year, each strain of the influenza virus is genetically distinct. In this sense, it is a constantly emerging pathogen.

You may also want to elaborate on the definition of emerging diseases by noting that this category includes (1) diseases that are truly "new" among humans (few, if any, examples fall into this subcategory); (2) diseases that probably affected a few individuals even hundreds and thousands of years ago, but have just recently affected enough of the population that they are noticed (AIDS and Ebola hemorrhagic fever are examples for this subcategory); and (3) diseases that affected people hundreds and thousands of years ago, but have just recently been recognized as due to an infectious pathogen (gastric ulcers caused by Helicobacter pylori is an example that falls into this subcategory). Many researchers include re-emerging diseases as a subcategory of emerging diseases.

Figure 17 - History of occurrence.
Emerging Diseases Re-emerging Diseases Endemic Diseases
AIDS, cholera, CJD, Ebola hemorrhagic fever, influenza, Legionnaire disease, Lyme disease tuberculosis, malaria, schistosomiasis pneumonia, polio, guinea worm disease, plague, strep throat

12. Conclude the activity by telling students that public health workers are becoming increasingly concerned about the emergence of "new" diseases and the re-emergence of some "old" diseases. These biologists have found it useful to classify infectious diseases as emerging, re-emerging, or endemic because there tend to be different factors related to each category. Tell students that they will explore factors related to disease emergence and re-emergence in upcoming activities.

Potential Extensions

Internet Web sites maintained by both the Centers for Disease Control and Prevention (www.cdc.gov/) and the World Health Organization (www.who.org/) include health topic sections that provide information on infectious (and noninfectious) diseases. Assign students to use these and other resources to create additional disease cards and to classify those diseases as emerging, re-emerging, or endemic.

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