The ultimate message in Parasites, Pathogens, and Progress is that geography, disease and economic development are all intertwined. The authors, Robert McGuire and Philip Coelho, use nineteenth-century U.S. history to substantiate this point. The authors combine several fields of research, including economic history, medical history, epidemiology, and genetics. Parasites echoes some of the general points made by Jared Diamond's Guns, Germs, and Steel (New York: W. W. Norton & Company, 1999), such as geography matters. However, the authors add new evidence and a provocative genetics angle. Parasites has two key theses: (1) inherited genetic resistance to disease helped determine the institution of slavery, and (2) disease feeds back negatively on economic development.

The first thesis centers on the institution of slavery. The authors argue the institution lasted in the South because slaves inherited genetic resistance to tropical diseases. This genetic resistance, in turn, made slaves relatively more productive laborers. For example, due to natural selection, West Africans were more likely to have the sickle cell trait, making them less vulnerable to malaria infections. In short, the diseases brought over from Africa helped entrap Africans as slaves in the American South. It is a provocative argument, which makes for an interesting read. To support this thesis, the authors rely on three strands of evidence. First, they argue that slavery could only have been profitable had the slaves been more productive laborers than indentured servants given their respective prices (Chapter 4). Second, after taking other factors into account, historical statistics suggest that African Americans in the South had relatively lower mortality rates and disease burdens than African Americans in the North and white Southerners (Chapter 5). Third, studies on contemporary African Americans suggest genetic resistance to tropical diseases (Chapter 6).

The second thesis is that models of economic growth should emphasize disease-related feedback mechanisms (Chapters 2 and 3). These mechanisms include population density, market size, and transportation costs/technology. For example, increased population density leads to more disease and death, a drain on economic development. Increased population density also leads to greater specialization, resulting in high fixed cost / low marginal cost forms of transportation (e.g., railroads). This increased population mobility leads to the proliferation of disease and death, resulting in a negative feedback on economic development. In support of this thesis, the authors correlate reductions in transportation costs throughout the nineteenth century to increases in tropical diseases in areas outside the South, like the Midwest (Chapter 7).

There is much to learn from reading Parasites. The discussions are accessible to readers who are not experts in the fields of economics, epidemiology, and/or genetics. For example, Chapter 2 (aptly titled “The Big Picture”) summarizes what is known about economic development and diseases throughout the history of mankind. Chapter 6 provides an excellent summary on how tropical diseases impact health capital accumulation. The authors’ arguments (or models) are developed clearly with flow diagrams, helping readers navigate the complex relationship between disease and economic development. Finally, the authors have compiled an immense amount of data and make it available in the Appendix for other researchers.

The book could have used more discussion in a few key areas. First, the two theses seem disjointed. For example, readers might have trouble finding a theme that unifies genetics and transportation costs. The unifying theme that “disease matters to history” is broad and has been made elsewhere (e.g., Diamond 1999). Focusing on first thesis, and dropping the discussion about transportation costs, would have improved the overall structure of the book. This would have kept the focus on the following memorable theme: geography determines genetics, disease, and (racist) institutions.

Second, the section on the genetics of contemporary African Americans is too brief (e.g., see malaria discussion on pp. 155–59). The authors summarize the basic facts well. For example, in contemporary populations, about “90 percent of West African and about 70 percent of African Americans lack the Duffy antigen,” a natural defense against the vivax strain of malaria (p. 157). However, it would be interesting to know whether genetic factors vary by geographic region of the United States. Was this influenced by slave trade patterns? Can geographic variation in genes possibly explain geographic differences in slave productivity in the nineteenth century? Given the provocative nature of the authors’ thesis, more background evidence from the genetics literature would have added to the book's appeal.

Third, a discussion on how “tipping points” enter the economic model seems warranted. Obviously, the South broke free from the Malthusian disease trap at some point in recent history. But, when and how did this come about? Was it due to technological innovations and/or changes in public health institutions? Or, was this due to structural changes in the agricultural sector? The authors acknowledge that “the story of how and why that linkage broke is an important one, but it is not our story” (p. 175). Of course, the authors rightfully make the focus of their study the nineteenth century. Still, some readers (e.g., development economists) may be disappointed by the lack of discussion here.

Finally, more background information on nineteenth-century attitudes about slaves’ disease resistance would help address concerns about biases in the data. It seems plausible that racism and prejudice may have influenced data collection. For example, were the hookworm samples collected blinded to race (pp. 142–43)? Alternatively, sample selection might have led to different composition of health capital across races. For example, along what other dimensions did the African American Union soldiers differ from their white counterparts (p. 141)?

In sum, I highly recommend Parasites, Pathogens, and Progress to economic and medical historians. The book is very approachable, so nonexperts and economists from a variety of backgrounds will find this an enlightening read.