Changes in diet and physical-activity patterns, population densities, and exposure to zoonoses associated with agriculture resulted in an epidemiological transition, a shift in the causes of morbidity (sickness) and mortality (death) among humankind (Omran 2005). The first epidemiological transition from foraging to food production resulted in increases in dental caries (see Chapter 12), nutritional deficiencies, infectious disease, and skeletal conditions like osteoarthritis, as well as decreases in growth and height (Larsen 2014). A second epidemiological transition occurred following the Industrial Revolution in Western Europe and the United States when improved standards of living, hygiene, and nutrition minimized the effects of infectious disease, after which people began to experience higher rates of noncommunicable diseases, such as cancer, heart disease, and diabetes due to the changes in lifestyle, diet, and activity levels that are the subject of this chapter (Omran 2005). With the addition of immunizations and other public health initiatives, modified forms of this transition remain ongoing in many low- and middle-income countries (Zuckerman et al. 2014), with several now facing a “double burden” of disease, with poor, often rural, populations struggling with infectious diseases due to malnutrition, lack of sanitation, and access to health care, while more affluent citizens are victims of chronic illnesses. A third epidemiological transition is now underway as infectious diseases, including new, re-emergent, and multidrug-resistant diseases, have once again become major health concerns (Harper and Armelagos 2010; Zuckerman et al. 2014). These include COVID-19, Ebola, HIV/AIDS, tuberculosis, malaria, dengue, Lyme disease, and West Nile virus—all zoonoses that initially spread to humans through contact with animals.
Patterns of morbidity and mortality continue to shift across the globe. As with the first epidemiological transition resulting from the adoption of large-scale agriculture, such shifts can be the direct, if unintended, result of human interactions with the environment. For example, there has been a rise in chronic inflammatory diseases (CIDs) in developed countries (Versini et al. 2015). This includes increased rates of allergic conditions like asthma and autoimmune diseases like rheumatoid arthritis, multiple sclerosis, Crohn’s disease, and inflammatory bowel disease. This has coincided with the decrease in infectious disease associated with the second epidemiological transition, and the two are related. The “hygiene hypothesis” postulates the rise in CIDs is a result of limited exposure to nonlethal environmental pathogens in utero and early childhood (Zuckerman and Armelagos 2014). Modern human societies have become so sanitized that we are no longer exposed to microorganisms that stimulate the development of a healthy immune system (Versini et al. 2015). “In effect, the lifestyle changes—sanitary improvements, pasteurization, use of antibiotics, and improved hygiene—that contributed to the second transition may have produced a substantial trade-off, with developed nations exchanging a high burden of infectious disease for a higher burden of CIDs” (Zuckerman et al. 2014).
The third epidemiological transition, the re-emergence of infectious disease, reflects the continuing relationship between humans, animals, and pathogens. Over 60% of emerging infectious diseases (EIDs) since 1940 have been of zoonotic origin, with over 70% stemming from human contact with wildlife (Jones et al. 2008), including COVID-19. The crossover of COVID to humans is believed to have involved transmission from bats to an intermediate species then to humans, with infected humans then passing it to other humans in a wet market in Wuhan, China in late 2019 (Worobey et al. 2022). Two COVID variants, representing two distinct crossover events from animals to humans, were circulating in the market by February 2020. Similarly, the global bushmeat trade currently devastating Africa’s wildlife is a continuing source of Ebola infection (Asher 2017), as well as the original source of HIV and viruses related to leukemia and lymphoma among humans (Zuckerman et al. 2014). New strains of avian (bird) flu, some with mortality rates as high as 60% among humans (WHO n.d.), are transmitted to humans through poultry production and contact with wild birds (Davis 2005). Lastly, the use of antibiotics in commercial meat production is directly related to the rise of drug-resistant strains of previously controlled infectious diseases. An estimated 80% of antibiotics in the U.S. are used to promote growth and prevent infection in livestock, and drug-resistant bacteria from these animals are transmitted to humans through meat consumption (Ventola 2015).
A fourth epidemiological transition is currently underway in which some parts of the globe are suffering from a “triple burden” of infectious and chronic diseases combined with injuries and diseases related to intensifying globalization, urbanization, deforestation, and climate change (Karn and Sharma 2021). Massive flooding in Pakistan in 2022 (Figure 16.3) will serve to illustrate the concept. Following a severe heat wave in June 2022, Pakistan experienced extremely heavy seasonal monsoon rains, in some provinces 700% above normal. Combined with water flow from melting glaciers, this caused the worst flooding in the country’s history, putting one third of the nation under water (Sheerazi 2022). The heat wave, glacial melt, and extreme rainfall were all attributable to global climate change, inflicting destruction and disease on Pakistan, which produces less than 1% of total global carbon emissions (Government of Pakistan 2021).
As a direct result of the flooding, infrastructure, including roads, homes, and bridges, was destroyed, and 1,700 people died, nearly 13,000 were injured, and over 33 million were displaced. In addition to their initial injuries and trauma, displaced people lacked food, health care, safe water, and basic sanitation, leading to starvation and exposure to infectious diseases like malaria and dengue fever, as well as skin conditions like scabies, caused by mites. Pakistan also has a poverty rate of 30–40%, contributing to already-high rates of HIV, tuberculosis, and hepatitis. At the same time, the leading causes of death are heart disease, cancer, lower respiratory diseases, and stroke (CDC n.d), all chronic conditions.
These examples illustrate continuing interaction between humans, our evolved biology, and the physical and cultural environments in which we live. The remainder of this chapter will focus on selected diseases and the social, cultural, and environmental factors that contribute to their prevalence in modern, industrialized economies. We begin with the health condition that affects all of the others—obesity.