Emerging & Re-emerging Infectiious Diseases
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Introduction

Emerging and Re-emerging Infectious Diseases has two objectives: to introduce students to major concepts related to emerging and re-emerging infectious diseases and to convey to students the relationship between basic biomedical research and the improvement of personal and public health. The improvement of personal and public health is the central mission of the National Institutes of Health, the world's largest organization devoted to biomedical research, and the funding agency for this module.

Figure 1 - Discovery of bacterial causes of several diseases.
Disease Year Discovered Scientist
anthrax
1876
Koch
gonorrhea
1879
Neisser
tuberculosis
1882
Koch
plague
1894
Kitasato, Yersin
whooping cough
1906
Bordet, Gengoe

In medieval times most people believed that supernatural forces created diseases to punish humankind for its sins (an idea that continues today in descriptions of AIDS as God's punishment of homosexuals and drug abusers). Nevertheless, as early as 1530, Gerolomo Frascatoro, an insightful Italian, suggested in a poem that syphilis and other diseases could be contagious—that is, they could be transmitted by direct contact with an infected person, contaminated materials, or infected air. The discovery of microorganisms by Anton van Leeuwenhoek in the late 1600s led some to speculate that these microscopic organisms might be the cause of disease. Although this "germ theory of disease" was first proposed in 1762, it was fully developed by Robert Koch in the 1870s as he studied anthrax, a disease of cattle and sometimes of humans. Koch devised a set of steps, now called Koch's postulates, to prove that a particular bacterium causes a specific disease: (1) The organism should always be found in animals suffering from the disease; (2) the organism must be isolated from the animal's body and cultivated in pure culture; (3) the culture should induce the same disease when inoculated into a healthy animal; and (4) the organism should be reisolated and cultured from the healthy animal and found to be the same as the original organism. Following Koch's initial work on anthrax, scientists identified the bacterial cause of many common diseases.

Despite great advances in determining the infectious agent involved in many bacterial diseases, the causes of many other diseases remained elusive. In 1898, Friedrich Loeffler and P. Frosch studied foot-and-mouth disease, a skin infection of animals. They discovered that the infectious agent for this disease was small enough to pass through filters that would screen out all known bacteria. Other experiments indicated that the causative agent was not a chemical toxin, but a "minute living being." In 1899, Martinus Beijerinck, a Dutch microbiologist who investigated the cause of tobacco mosaic disease in tobacco and tomato plants, proposed that the infectious agent was a "filterable virus" that must be incorporated into cells in order to reproduce. In 1900, Walter Reed discovered that yellow fever in humans is caused by a virus. The work of these and other researchers led to an understanding of the viral basis of many diseases. The development of more sophisticated biochemical techniques in the early 1900s revealed the chemical simplicity of viruses (consisting of just protein and nucleic acid), and the invention of the electron microscope in 1932 allowed viruses to be visualized.

In addition to bacteria and viruses, physicians recognized that some infectious diseases are caused by fungi, protozoa, and helminths from the roundworm and flatworm phyla. Protozoa and helminths are sometimes collectively called "parasites," meaning organisms that live at the expense of another organism (termed "the host"). Technically, infectious bacteria and viruses could also be considered parasites. Recent evidence indicating that some neurological disorders are due to infection by unusual proteins, named prions, suggests that other types of pathogens may also exist.

Even as scientists began to understand the microbial cause of infectious diseases, medical workers were searching for ways to prevent or treat these diseases. For example, physicians had long known that survivors of many infectious diseases were immune to further infection by the disease-causing agent. For centuries, the Chinese had used variolization (introducing dried material from smallpox lesions into scratches on a healthy individual's skin) to induce a mild smallpox infection that would prevent the individual from contracting a severe or lethal case later in life. This procedure spread through Asia and was eventually introduced to the European community. Unfortunately, variolization occasionally caused severe and even lethal cases of smallpox. In 1798, however, the rural English physician Edward Jenner made a curious observation. His patients who had contracted and recovered from cowpox, a disease similar to but much milder than smallpox, seemed to be immune not only to further cases of cowpox, but also to smallpox. By scratching the fluid from cowpox lesions into the skin of healthy individuals, he was able to immunize those people against smallpox. Louis Pasteur later developed vaccines for anthrax (caused by a type of bacterium) and rabies (caused by a virus) by treating the infectious agents for those diseases so that they lost their disease-producing abilities. Vaccination is now used to immunize people against many diseases.

Biologists also identified conditions and chemical agents that killed bacteria, leading to the prevention of many diseases. Pasteur used heat to sterilize culture media, eliminating unwanted microorganisms. The process of pasteurization, named in his honor, is now used to kill bacteria in a variety of beverages. Joseph Lister sprayed surgical rooms with aqueous phenol to reduce wound infections. People also began to recognize the importance of clean water and of treating sewage for preventing disease.

A key step forward in the fight against infectious disease was the discovery and development of drugs that could kill the microbe involved without killing the patient. Antibacterial drugs were discovered first. In the 1930s, Gerhard Domagk discovered that Prontosil, a sulfonamide, could cure streptococcal infections in mice. In 1929, Alexander Fleming discovered that a substance produced by the mold penicillium killed cultures of staphylococcal bacteria. He characterized the product and named it penicillin. Later, in the early 1940s, a group of British scientists directed by Howard Florey showed that penicillin was effective in controlling some infectious diseases and developed procedures for its mass production. The pharmaceutical industry flourished after World War II, and many additional antibiotics were discovered or synthesized.

Developing antiviral drugs has been more challenging. Because viruses reproduce inside host cells, it is difficult to find drugs that interfere with viral reproduction but are not toxic to host cells. Most of the drugs used today interfere with the enzymes involved in viral replication and do not affect (or affect only slightly) enzymes that are essential for the host cell. Acyclovir, used to treat genital herpes, and amantadine, used to prevent influenza A, are two examples of drugs that interfere with viral replication. AZT, the first drug to be widely used in the treatment of AIDS, also interferes with viral replication. In contrast, the newer protease inhibitors used to treat AIDS interfere with the process of virus packaging. Antifungal, antiprotozoan, and antihelminthic drugs also have been discovered; these drugs frequently have serious side effects and must be administered carefully.

Science and medicine have made dramatic advances across the last two centuries in understanding, preventing, and treating infectious diseases. Despite these advances, the last two decades have witnessed the emergence of a number of previously unrecognized diseases and the re-emergence of several previously well-controlled ones. This phenomenon is intriguing from a biological standpoint, but is alarming from a public health standpoint.

In this module, students explore the biological factors associated with disease emergence and re-emergence and consider the human activities that can increase or decrease the likelihood of outbreaks of infectious diseases. There are many concepts we could have addressed, but we have chosen, with the help of a variety of experts in this field, a relatively small number for your students to explore. Those concepts follow.

We hope the five activities provided in this module (Figure 2) will be effective vehicles to carry these concepts to your students. Although the activities contain much interesting information about specific infectious diseases, we suggest that you focus your students' attention on the major concepts the module was designed to convey. The concluding steps in each activity are intended to focus the students' attention on these concepts as the activity draws to a close.

Figure 2 - This diagram identifies the module's major sections and describes their contents.
Activity 1
Deadly Disease Among Us
Activity 2
Disease Detectives
Activity 3
Superbugs: An Evolving Concern
Activity 4
Protecting the Herd
Activity 5
Making Hard Decisions
Students take a short quiz on the impact of infectious diseases, then classify several diseases as "emerging," "re-emerging," or "endemic." Students assume the roles of public health experts to investigate the cause of a mystery disease. Students investigate the growth of bacteria in the presence of antibiotics and use the results to explain a case of antibiotic-resistant tuberculosis. Students use simulations of the spread of an infectious disease to discover the phenomenon of herd immunity. Students evaluate proposals to combat three infectious diseases and recommend one to support.
 

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