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.
Of course, the absence of crowd diseases from small populations of hunter-gatherers does not mean that hunter-gatherers are free from infectious diseases. They do have infectious diseases, but their diseases are different from the crowd diseases in four respects. First, the microbes causing their diseases are not confined to the human species but are shared with animals (such as the agent of yellow fever, shared with monkeys) or else capable of surviving in soil (such as the agents causing botulism and tetanus). Second, many of the diseases are not acute but chronic, such as leprosy and yaws. Third, some of the diseases are transmitted inefficiently between people, leprosy and yaws again being examples. Finally, most of the diseases do not confer permanent immunity: a person who has recovered from one bout of a disease can contract the same disease again. These four facts mean that these diseases can maintain themselves in small human populations, infecting and re-infecting victims from animal and soil reservoirs and from chronically sick people.
Hunter-gatherers and small farming populations are not immune to crowd diseases; they are merely unable to maintain crowd diseases by themselves. In fact, small populations are, tragically, especially susceptible to crowd diseases when they become infected by a visitor from the outside world. Their enhanced susceptibility is due to the fact that at least some of the crowd diseases tend to have higher fatality rates in adults than in children. In dense urban First World populations everyone (until recently) became exposed to measles as a child, but in a small isolated population of hunter-gatherers the adults have not been exposed to measles and are likely to die of it if it arrives. There are many horror stories of Inuit, Native American, and Aboriginal Australian populations being virtually wiped out by epidemic diseases introduced through European contact.
Responses to diseases
For traditional societies, diseases differ from the other three major types of dangers as regards people’s understanding of the underlying mechanisms, and hence of effective cures or preventive measures. When someone is injured or dies from an accident, violence, or hunger, the cause and underlying process are clear: the victim was hit by a falling tree, struck by an enemy’s arrow, or starved by insufficient food. The appropriate cure or preventive measure is equally clear: don’t sleep under dead trees, watch out for enemies or kill them first, and ensure a reliable food supply. However, in the case of diseases, sound empirical understanding of causes, and science-based preventive measures and cures, achieved notable success only within the last two centuries. Until then, state societies as well as traditional small-scale societies suffered heavy tolls from disease.
This is not to say that traditional peoples have been completely helpless at preventing or curing diseases. The Siriono evidently understand that there is a connection between human feces and diseases such as dysentery and hookworm. A Siriono mother promptly cleans up her infant’s feces when it defecates, stores the feces in a basket, and eventually dumps the basket’s contents far away in the forest. But even the Siriono are not rigorous in their hygiene. Anthropologist Allan Holmberg relates watching a Siriono infant unobserved by his mother defecate, lie in his feces, smear them over himself, and put them into his mouth. When his mother finally noticed what was going on, she put her finger into the baby’s mouth, removed the feces, wiped but didn’t bathe the filthy baby, and resumed eating herself without washing her hands. Piraha Indians let their dogs eat off the plates from which they themselves are simultaneously eating: that’s a good way to acquire canine germs and parasites.
By trial and error, many traditional peoples identify local plants which they believe help cure particular ailments. My New Guinea friends frequently point out to me certain plants which they say that they use to treat malaria, other fevers, or dysentery or to induce miscarriage. Western ethnobotanists have studied this traditional pharmacological knowledge, and Western pharmaceutical companies have extracted drugs from these plants.
