Among diseases, the relative importance of different categories of disease for traditional peoples varies greatly with lifestyle, geographic location, and age. In general, infectious diseases are most important among infants and young children and remain important at all ages. Parasitic diseases join infectious diseases in importance in childhood. Diseases associated with worm parasites (such as hookworm and tapeworm) and insect-born protozoan parasites (such as malaria and the agent causing sleeping sickness) are more of a problem for peoples of warm tropical climates than for peoples of the Arctic, deserts, and cold mountaintops, where the worms themselves and the protozoa’s insect vectors have difficulty surviving in the environment. Later in life, degenerative diseases of bones, joints, and soft tissue—such as arthritis, osteoarthritis, osteoporosis, bone fractures, and tooth wear—rise in importance. The much more physically demanding lifestyle of traditional peoples than of modern couch potatoes makes the former more susceptible than the latter to such degenerative diseases at a given age. Conspicuously rare or absent among traditional peoples are all of the diseases responsible for most deaths in the First World today: coronary artery disease and other forms of atherosclerosis, stroke and other consequences of hypertension, adult-onset diabetes, and most cancers. I shall discuss the reasons for this striking difference between First World and traditional health patterns in Chapter 11.
Only within the last two centuries have infectious diseases receded in importance in the First World as causes of human death. The reasons for those recent changes include appreciation of the importance of sanitation; the installation of clean water supplies by state governments, the introduction of vaccination, and other public health measures; the growth of scientific knowledge of microbes as the agents of infectious disease, permitting rational design of effective counter-measures; and the discovery and design of antibiotics. Poor hygiene permitted (and still permits today) the transmission of infectious and parasitic diseases among traditional peoples, who often use the same water supply for drinking, cooking, bathing, and washing, defecate nearby, and do not understand the value of washing one’s hands before handling food.
Just to mention an example of hygiene and disease that impressed me personally, on a trip to Indonesia during which I spent most of each day bird-watching alone on forest trails radiating from a campsite shared with Indonesian colleagues, I was disconcerted to discover that I was experiencing sudden attacks of diarrhea at an hour varying unpredictably from day to day. I racked my brain to figure out what I was doing wrong, and what could account for the variation of the attacks’ timing. Finally, I made the connection. Each day, a wonderfully kind Indonesian colleague, who felt responsible for my well-being, came out from camp and followed my trail of that day until he encountered me, to make sure that I hadn’t had an accident or gotten lost. He handed me some biscuits that he had thoughtfully brought from camp as a snack, chatted with me for a few minutes to satisfy himself that all was well with me, and returned to camp. One evening, I suddenly realized that my diarrhea attack each day began about half an hour after my kind friend had met me and I had eaten his biscuits on that day: if he met me at 10:00 A.M., my attack came at 10:30, and if he met me at 2:30 P.M., it came at 3:00 P.M. From the next day onwards, I thanked him for his biscuits, disposed of them inconspicuously after he had turned back, and never had any more attacks. The problem had originated with my friend’s handling of the biscuits rather than with the biscuits themselves, of which we kept a supply in their original cellophane packets at our camp, and which never made me ill when I opened the packet myself. Instead, the cause of the attacks must have been intestinal pathogens transmitted from my friend’s fingers to the biscuits.
The prevalent types of infectious diseases differ strikingly between small populations of nomadic hunter-gatherers and family-level farming societies on the one hand, and large populations of modern and recently Westernized societies plus traditional densely populated Old World farming societies on the other hand. Characteristic diseases of hunter-gatherers are malaria and other arthropod-transmitted fevers, dysentery and other gastrointestinal diseases, respiratory diseases, and skin infections. Lacking among hunter-gatherers, unless they have been recently infected by Western visitors, are the feared infectious diseases of settled populations: diphtheria, flu, measles, mumps, pertussis, rubella, smallpox, and typhoid. Unlike the infectious diseases of hunter-gatherers, which are present chronically or else flare up and down, those diseases of dense populations run in acute epidemics: many people in an area become sick within a short time and quickly either recover or die, then the disease vanishes locally for a year or more.
The reasons why those epidemic diseases could arise and maintain themselves only in large human populations have emerged from epidemiological and microbiological studies of recent decades. Those reasons are that the diseases are efficiently transmitted, have an acute course, confer lifetime immunity on victims who survive, and are confined to the human species. The diseases become transmitted efficiently from a sick person to nearby healthy people by microbes that a patient excretes onto his skin from oozing pustules, that a patient ejects into the air by coughing and sneezing, or that enter nearby water bodies when a patient defecates. Healthy people become infected by touching a patient or an object handled by the patient, breathing in the patient’s exhaled breath, or drinking contaminated water. The disease’s acute course means that, within a few weeks of infection, a patient either dies or recovers. The combination of efficient transmission and acute course means that, within a short time, everybody in a local population has become exposed to the disease and is now either dead or recovered. The lifetime immunity acquired by survivors means that there is no one else alive in the population who could contract the disease until some future year, when a new crop of unexposed babies has been born. Confinement of the disease to humans means that there is no animal or soil reservoir in which the disease could maintain itself: it dies out locally and cannot come back until an infection spreads again from a distant source. All of those features in combination mean that these infectious diseases are restricted to large human populations, sufficiently numerous that the disease can sustain itself within the population by moving constantly from one area to another, locally dying out but still surviving in a more distant part of the population. For measles the minimum necessary population size is known to be a few hundred thousand people. Hence the diseases can be summarized as “acute immunizing crowd epidemic infectious diseases of humans”—or, for short, crowd diseases.
The crowd diseases could not have existed before the origins of agriculture around 11,000 years ago. Only with the explosive population growth made possible by agriculture did human populations reach the high numbers required to sustain our crowd diseases. The adoption of agriculture enabled formerly nomadic hunter-gatherers to settle down in crowded and unsanitary permanent villages, connected by trade with other villages, and providing ideal conditions for the rapid transmission of microbes. Recent studies by molecular biologists have demonstrated that the microbes responsible for many and probably most of the crowd diseases now confined to humans arose from crowd diseases of our domestic animals such as pigs and cattle, with which we came into regular close contact ideal for animal-to-human microbe transfer only upon the beginnings of animal domestication around 11,000 years ago.