Before beginning this assignment, read Chapters 3 and 4 (Signs and Symptoms of Infectious Disease and An Arms Race Without End) in Why We Get Sick (Nesse and Williams). Then choose one of the two problems below and write a one-page essay that addresses the points raised in your selected problem. Once again, the intended audience for your essay is intelligent and interested readers who are unfamiliar with Darwinian medicine and evolutionary biology. Your job is to inform these readers, as best you can, in your one-page essay.

Your essay must be computer-printed. Although the choice of font style and margins is up to you, your essay must be double-spaced, in 10-point or larger font, and fit on a single page. Submit it attached to a cover sheet that has your name, honor pledge, and date.

1. In Chapter 3, Nesse and Williams classify of the signs and symptoms of infectious disease into three categories: phenomena that are evolved defenses that benefit the host, phenomena that are incidental consequences of the host-pathogen interaction and benefit neither party, and phenomena that benefit the pathogen and may represent instances of host manipulation, in which a pathogen has evolved to alter the physiological machinery of the host to serve its own interests (see, for example, the discussion of rabies on p. 46).

For many of the signs and symptoms of infectious diseases, drawing clear distinctions among evolved defense, incidental consequence, and host manipulation can be difficult. Cholera, for example, is a disease caused by intestinal infections of the bacteria Vibrio cholerae and spread by consumption of contaminated water. One of the major symptoms of cholera is severe diarrhea.

In your essay, do the following: (A) Propose a hypothesis that would explain the severe diarrhea associated with cholera as an evolved defense that benefits the human host. (B) Propose a hypothesis that would explain the severe diarrhea as a case of host manipulation that benefits the cholera pathogen. (C) Briefly describe a clinical study or experiment that would allow you to test and discriminate between the defense and manipulation hypotheses. (D) Describe the potential clinical implications of your findings. In other words, how might the results of your study alter the way in which cholera is treated?

2. Imagine two human populations afflicted with the HIV virus and AIDS. In Population A, most people are sexually promiscuous, and a sexually active individual may have several different partners over the course of a year. In addition, "safe sex" practices such as the use of condoms are seldom employed. In Population B, in contrast, sexual promiscuity is generally viewed negatively by the population. Most sexually active individuals have only 1-2 partners over the course of a year, and condom use is widespread.

Obviously, the frequency (incidence of occurrence) of HIV and AIDS will be higher in Population A than in Population B. But what can you predict about the relative virulence of the HIV virus in the two populations? Explain your prediction and your reasoning as completely and clearly as you can in your one-page essay, and be sure to use language that accurately reflects the underlying principles of natural selection and evolutionary biology.