Nobody comprehends the universe, because it is almost entirely out of sight. We also can’t see the universe of microorganisms here on Earth, or fully comprehend their powerful influence. Historian William McNeill learned that disease has played a major role in the human journey, and he wrote a fascinating introduction to our intimate companions, the parasites, in Plagues and Peoples.
All critters eat. Hosts provide food, and parasites consume it. Large-bodied parasites, like wolves, are macro-parasites. Wolves kill their hosts. Micro-parasites include bacteria, viruses, and small multi-celled organisms. If they quickly kill their host, the banquet is short. A more stable strategy is to simply take a free ride on a living host, like the billions of bacteria that inhabit our guts, share our meals, and don’t make us sick.
In healthy ecosystems, stability is the norm. Species coevolve, which encourages balance, like the dance of oak trees and squirrels, or the foxes and rabbits. Balance is disturbed by natural disasters, like when an invasion of organic farmers overwhelms an ecosystem with their plows, axes, and enslaved animals. A farming community is a mob of macro-parasites that weakens or destroys its ecosystem host over time. When parasites disturb balance, McNeill calls this disease. “It is not absurd to class the ecological role of humankind in its relationship to other life forms as a disease.”
The ruling classes in civilizations behave like macro-parasites when they siphon nutrients away from the working class hosts that they exploit. To survive, the elites must keep enough farmers alive to maintain an adequate supply of nutrients. Elites rely on violence specialists to protect their host collection from other two-legged macro-parasites, like the bloodthirsty civilization across the river. In this scenario, the worker hosts are suffering from a type of disease (the elites) that is called endemic, because it allows them to survive.
Disease that kills the host is epidemic. “Looked at from the point of view of other organisms, humankind therefore resembles an acute epidemic disease, whose occasional lapses into less virulent forms of behavior have never yet sufficed to permit any really stable, chronic relationship to establish itself.”
Our chimp and bonobo cousins continue to have a stable relationship with their ecosystem. Consequently, there are not seven billion of them. Like them, our pre-human ancestors evolved in a tropical rainforest, a warm and wet ecosystem with immense biodiversity. This diversity included many, many types of parasites, and they lovingly helped to keep our ancestors in balance. Life was good. “The balance between eater and eaten was stable, or nearly so, for long periods of time.”
Then, some too-clever ancestors began fooling around with technology. With spears, we were able to kill more prey, and foolishly eliminate many of the rival predators that helped keep our numbers in check. By and by, our ancestors began leaving Africa, moving into cooler and drier climates. We left behind many tropical parasites, and explored new lands with far fewer parasites. We suffered less disease. We moved into new regions as skilled hunters, and encountered game animals that had no fear of us. With clever new technology, like clothing and huts, our ancestors could sidestep our biological limitations and survive in non-tropical habitats.
Antelope and tsetse flies are unaffected by the sleeping sickness parasites they carry. Many species of burrowing rodents live with the bubonic plague bacteria harmlessly. These relationships are old and stable, but a blind date with a new parasite can be fatal. With the advent of animal domestication, there were many blind dates. We began living in close proximity to other species, and their parasites, to which we had no immunity. This gave birth to the deadly new diseases of civilization, and led to a long era of epidemics.
“Most and probably all of the distinctive infectious diseases of civilization transferred to human populations from animal herds.” Aborigines, who did not enslave herd animals, did not suffer from infectious disease. The same was true for Native Americans, even those who lived in the densely populated regions of Mexico, Central America, and the Andes.
Humans share many diseases with domesticated animals: poultry (26), rats and mice (32), horses (35), pigs (42), sheep and goats (46), cattle (50), and dogs (65). In addition to the diseases of civilization are ancient rainforest diseases like malaria and yellow fever, which were introduced to the Americas by the slave trade.
From 500 B.C. to A.D. 1200, as civilizations developed in different regions of Eurasia, each area developed pools of civilized diseases, some of which became quite popular. India has a wonderful climate for parasites, and it may be where smallpox, cholera, and plague parasites first entered human hosts. Bubonic plague slammed into a virgin population in the Mediterranean basin. The plague of Justinian (A.D. 542-543) hit hard, maybe killing 100 million, about half of Europe.
From 1200 to 1500, the isolated disease pools of Eurasia eventually connected with the others, creating one large pool of civilized diseases. Nomads, like the Mongols, transported parasites back and forth between China and Europe. Parasites also travelled by ship. Black Death began in China around 1331. Between 1200 and 1393, China’s population dropped by half. The disease arrived in Crimea in 1346, spread across Europe, and killed about a third of the people. Muslims believed that those killed by the plague were martyrs, chosen by the will of Allah. They mocked the Christian infidels who successfully limited the spread via quarantines.
Between 1300 and 1700, a number of epidemic diseases became domesticated. To survive, parasites required a steady supply of new hosts without immunity — these were mostly children. A population of 500,000 or more was needed to produce enough new hosts to support an ongoing infestation of measles. If a disease was too virulent, it would eliminate its hosts and die off. Over time, a number of serial killers softened into childhood diseases, like mumps, smallpox, and measles.
From 1500 to 1700, Old World diseases discovered the New World. Europeans and their African slaves were walking disease bombs, but they were mostly immune to the parasites they carried. Native Americans were a virgin population, having no immunity whatsoever to the new parasites, they were blindsided by catastrophic epidemics. The population of Mexico and Peru dropped 90 percent in 120 years.
Since 1700, science has made great advances in death control (not balanced by equal achievements in birth control). Vaccinations have been effective in controlling smallpox and polio. Antibiotics have temporarily provided several decades of relief from a number of infectious parasites. Sewage treatment and water purification systems have also provided temporary relief, during the bubble of abundant energy.
Industrial society, with its radically unhealthy way of life, has created new diseases of civilization, like cancer and heart disease. Influenza is a powerful wild card, because it rapidly mutates, sometimes into highly virulent forms. By the time the vaccines are mass-produced, the pandemic is over. Many new viral diseases, like Ebola and AIDS, are appearing, as the human swarm meets new and exciting rainforest parasites.
The plague bacterium still lives harmlessly in burrowing rodents and their fleas. Over the years, it has spread around the world. By 1940, it was carried by 34 species of burrowing rodents in America, and 35 species of fleas. By 1975, it was found across the western U.S., and portions of Canada and Mexico. Black rats are the vector that moves the parasites into humans. As long as the gas-guzzling garbage trucks keep running regularly, we’ll be safe, maybe.
Modern consumers have had little exposure to epidemic disease, but our elaborate, energy-guzzling systems of death control only provide temporary protection. Sewage treatment, water purification, effective antibiotics, and industrial agriculture have a limited future in a Peak Energy world.
McNeill, William H., Plagues and Peoples, Anchor Books, New York, 1998. 
Other reviews of books on health include: Bird Flu, Epidemics, Health & the Rise of Civilization, The Antibiotic Paradox.